Yeah it generates a cia file! sorry should have mentioned. It downloads everything from CDN and converts it straight into a cia file just like what yours can do. (I use it with makerom)(the name is weird, it sounds like it takes a 3ds and puts it on cdn, of course not though!)
do you mean CDNto3DS from here? https://github.com/Relys/3DS_Multi_Decryptor/tree/master/to3DS
it's not a 3ds file - anyway, the problem is make_cdn_cia, not the python file etc - the cia generator, makerom has the same problem i'm pretty sure.
I had a go at it and made a 74 content pack fine