First Try

overlays/10-feather-font.nix

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+self: super: with super; {
+
+  feather-font = let
+    version = "1.0";
+    pname = "feather-font";
+  in stdenv.mkDerivation {
+    name = "${pname}-${version}";
+
+    src = fetchzip {
+      url = "https://github.com/dustinlyons/feather-font/archive/refs/tags/${version}.zip";
+      sha256 = "sha256-Zsz8/qn7XAG6BVp4XdqooEqioFRV7bLH0bQkHZvFbsg=";
+    };
+
+    buildInputs = [ unzip ];
+    phases = [ "unpackPhase" "installPhase" ];
+
+    installPhase = ''
+      mkdir -p $out/share/fonts/truetype
+      cp $src/feather.ttf $out/share/fonts/truetype/
+    '';
+
+    meta = with lib; {
+      homepage = "https://www.feathericons.com/";
+      description = "Set of font icons from the open source collection Feather Icons";
+      license = licenses.mit;
+      maintainers = [ maintainers.dlyons ];
+      platforms = [ platforms.x86_64-linux platforms.x86_64-darwin ];
+    };
+  };
+}

overlays/README.md

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+# Overlays
+
+Files in this directory run automatically as part of each build. Some common ways I've used overlays in the past:
+* Applying patches
+* Downloading different versions of files (locking to a version or trying a fork)
+* Workarounds and stuff I need to run temporarily