Flake input is unsupported, how can I create a dummy flake?

I’ve a Flake with several inputs (none of them is a flake itself), some of which as url have a specific GitHub release tarball, e.g.

katex-src = {
  url = "https://github.com/KaTeX/KaTeX/releases/download/v0.12.0/katex.tar.gz";
  flake = false;

I also need as inputs the .css and .js files released here.
The files are not present in the zip and tar.gz tarballs since they are built by the CD workflow.
If I try to use the js or css links it gives me the error input [...] is unsupported.
One solution could be create a local directory with a custom flake building (or simply fetching) these files, then I could add an input to my main flake with as url something like file:// (I found the possibilities to use these urls here).
Could I do something like this without creating a subfolder with a git repository in my project or even an external GitHub repository?
My Nix language skills are lacking, maybe is there a way to declare a dummy flake in a let expression and then add it to my inputs?

Thank you!

You need to manage your non flake dependencies outside of the inputs:

Flake inputs

The attribute inputs specifies the dependencies of a flake, as an attrset mapping input names to flake references. For example, the following specifies a dependency on the nixpkgs and import-cargo repositories:

In a broader extend you can use any nix expression by specifying flake = false, though it still has to be valid nix expression as it is “evaluated before beeing passed to the outputs function” (mentioned somewhere else on the same page).

So you need to manage your non-nix dependencies by other means.

You can use niv or just nix-prefetch-* to maintain a list of JSON files that specify inputs etc.