diff --git a/src/platform/am1i/custom.fmap b/src/platform/am1i/custom.fmap
deleted file mode 100644
index a4f753d..0000000
--- a/src/platform/am1i/custom.fmap
+++ /dev/null
@@ -1,18 +0,0 @@
-#
-# Note: on x86 platforms the SPI flash is mapped into or copied into
-# the topmost X bytes of memory, and the very topmost word of memory
-# is the "reset vector" which points to the BIOS entry point.  Because
-# of this we must protect the TOPMOST half of memory; if an attacker
-# controls the reset vector and any other chunk of the flash, the game
-# is over.
-#
-FLASH@0 0x1000000 {
-	BIOS@0 0x1000000 {
-                # read-write zone
-		NORMAL(CBFS)   @    0x400 0x7FFC00
-
-                # read-only zone (eventually)
-		FMAP           @ 0x800000    0x400
-		FALLBACK(CBFS) @ 0x800400 0x7FFC00
-	}
-}
diff --git a/src/platform/am1i/default.nix b/src/platform/am1i/default.nix
index 2cf908f..c222d65 100644
--- a/src/platform/am1i/default.nix
+++ b/src/platform/am1i/default.nix
@@ -19,7 +19,26 @@
 
     console-device = "ttyS1";
     payload = "${final.kernel}/bzImage";
-    fmap = ./custom.fmap;
+    fmap = final.nixpkgsOnBuildForBuild.writeText "custom.fmap" ''
+      #
+      # Note: on x86 platforms the SPI flash is mapped into or copied into
+      # the topmost X bytes of memory, and the very topmost word of memory
+      # is the "reset vector" which points to the BIOS entry point.  Because
+      # of this we must protect the TOPMOST half of memory; if an attacker
+      # controls the reset vector and any other chunk of the flash, the game
+      # is over.
+      #
+      FLASH@0 0x1000000 {
+        BIOS@0 0x1000000 {
+          # read-write zone
+          NORMAL(CBFS)   @    0x400 0x7FFC00
+
+          # read-only zone (eventually)
+          FMAP           @ 0x800000    0x400
+          FALLBACK(CBFS) @ 0x800400 0x7FFC00
+        }
+      }
+    '';
 
     coreboot = (prev.coreboot.override {
       iasl = final.iasl_20180531;
@@ -30,12 +49,7 @@
         VENDOR_ASUS = lib.mkForce yes;
         BOARD_ASUS_AM1I_A = lib.mkForce yes;
 
-        COREBOOT_ROMSIZE_KB_16384 = lib.mkForce yes;
-        COREBOOT_ROMSIZE_KB = lib.mkForce (freeform (toString (16 * 1024)));
-        ROM_SIZE = lib.mkForce (freeform "0x1000000");
-
         CBFS_SIZE = lib.mkForce (freeform "0x7FFAC8");
-
         CONSOLE_CBMEM = lib.mkForce no;
         DRIVERS_INTEL_WIFI = lib.mkForce no;
         HUDSON_XHCI_ENABLE = lib.mkForce no;
diff --git a/src/platform/kevin/custom.fmap b/src/platform/kevin/custom.fmap
deleted file mode 100644
index aef09f4..0000000
--- a/src/platform/kevin/custom.fmap
+++ /dev/null
@@ -1,22 +0,0 @@
-# layout for firmware when flash address space matches used address layout
-# +-------------+ <-- 0
-# | unspecified |
-# +-------------+ <-- BIOS_BASE
-# | bootblock   |
-# +-------------+ <-- BIOS_BASE + 128K
-# | FMAP        |
-# +-------------+ <-- BIOS_BASE + 128K + FMAP_SIZE
-# | CBFS        |
-# +-------------+ <-- ROM_SIZE
-
-FLASH@0 0x1000000 {
-                # read-only zone (eventually)
-		BOOTBLOCK                     128K
-		FMAP           @  0x20000    0x400
-                # gap here from   0x20400 to keep FALLBACK 4k-aligned
-		FALLBACK(CBFS) @  0x21000 0x7DF000
-
-                # read-write zone
-		NORMAL(CBFS)   @ 0x800000 0x7DF000
-                MISC           @ 0xFDF000  0x21000    # 132k
-}
diff --git a/src/platform/kevin/default.nix b/src/platform/kevin/default.nix
index e9a7d43..aea584d 100644
--- a/src/platform/kevin/default.nix
+++ b/src/platform/kevin/default.nix
@@ -75,7 +75,30 @@ in {
     };
 
     payload = "${final.fit}/Image.fit";
-    fmap = ./custom.fmap;
+    fmap = final.nixpkgsOnBuildForBuild.writeText "custom.fmap" ''
+      # layout for firmware when flash address space matches used address layout
+      # +-------------+ <-- 0
+      # | unspecified |
+      # +-------------+ <-- BIOS_BASE
+      # | bootblock   |
+      # +-------------+ <-- BIOS_BASE + 128K
+      # | FMAP        |
+      # +-------------+ <-- BIOS_BASE + 128K + FMAP_SIZE
+      # | CBFS        |
+      # +-------------+ <-- ROM_SIZE
+
+      FLASH@0 0x1000000 {
+          # read-only zone (eventually)
+          BOOTBLOCK                     128K
+          FMAP           @  0x20000    0x400
+          # gap here from   0x20400 to keep FALLBACK 4k-aligned
+          FALLBACK(CBFS) @  0x21000 0x7DF000
+
+          # read-write zone
+          NORMAL(CBFS)   @ 0x800000 0x7DF000
+          MISC           @ 0xFDF000  0x21000    # 132k
+      }
+    '';
 
     image = prev.image.override {
       initramfs = null;  # it is part of the FIT image
diff --git a/src/platform/kgpe/custom.fmap b/src/platform/kgpe/custom.fmap
deleted file mode 100644
index a09635e..0000000
--- a/src/platform/kgpe/custom.fmap
+++ /dev/null
@@ -1,17 +0,0 @@
-#
-# Note: on x86 platforms the SPI flash is copied into the topmost X
-# bytes of memory, and the very topmost word of memory is the "reset
-# vector" which points to the BIOS entry point.  Because of this we
-# must protect the TOPMOST half of memory; if an attacker controls the
-# reset vector and any other chunk of the flash, the game is over.
-#
-FLASH@0 0x1000000 {
-	BIOS@0 0x1000000 {
-                # read-write zone
-		NORMAL(CBFS)   @    0x400 0x7FFC00
-
-                # read-only zone (eventually)
-		FMAP           @ 0x800000    0x400
-		FALLBACK(CBFS) @ 0x800400 0x7FFC00
-	}
-}
diff --git a/src/platform/kgpe/default.nix b/src/platform/kgpe/default.nix
index 9913fba..8956667 100644
--- a/src/platform/kgpe/default.nix
+++ b/src/platform/kgpe/default.nix
@@ -43,7 +43,25 @@
     cmos-default = null;
 
     payload = "${final.kernel}/bzImage";
-    fmap = ./custom.fmap;
+    fmap = final.nixpkgsOnBuildForBuild.writeText "custom.fmap" ''
+      #
+      # Note: on x86 platforms the SPI flash is copied into the topmost X
+      # bytes of memory, and the very topmost word of memory is the "reset
+      # vector" which points to the BIOS entry point.  Because of this we
+      # must protect the TOPMOST half of memory; if an attacker controls the
+      # reset vector and any other chunk of the flash, the game is over.
+      #
+      FLASH@0 0x1000000 {
+        BIOS@0 0x1000000 {
+          # read-write zone
+          NORMAL(CBFS)   @    0x400 0x7FFC00
+
+          # read-only zone (eventually)
+          FMAP           @ 0x800000    0x400
+          FALLBACK(CBFS) @ 0x800400 0x7FFC00
+        }
+      }
+    '';
 
     coreboot = (prev.coreboot.override {
       iasl = final.iasl_20180531;