Change Content-Length header on Workers

Hello! I am trying to change the Content-Length header, as I will use Cloudflare Workers to act as a middleman between the client and Cloudflare R2. It will serve small (<10Mb) .bin files to the client only after authentication. I have tried by changing the header manually, but it doesnt make a change. Then I read that it was not possible to change that header, and that the only way to do so is by using Streams or by Buffering the file.

Now, my question is how should I do that? I tried to do it (see the code below), however it does not work. Any advice is very much appreciated!

firmwareBin = await env.R2.get(decodeURI(key));
let { readable, writable } = new FixedLengthStream(175104);
return new Response(readable, firmwareBin);