There's a special thing for handling fonts, you basically need ~20 lines of boilerplate to fetch an archive with the .ttf files in, and this code does the rest.
Input Mono is gratis for personal use, but if you publish anything that uses it, you need to pay for a license. It's not freely redistributable.
Not freely redistributable shouldn't be a problem! Guix will download the fonts when someone requests an install, which isn't redistribution.
Except.
The hash of the .zip of the fonts is different every time you download it. I don't know why, but the actual .ttf files are ~6 bytes different in every download. Doesn't seem like enough data for it to be a fingerprint, but, maybe?
Guix has a strong focus on reproduceability, you have to provide the hash of any source material. Which you cannot do if the source material is different every download.
I asked for help, it seems like it's literally impossible to handle this, unless I download a copy and host it elsewhere, then point Guix at that and give it the hash.
Which is redistribution, which is against the license.
So I can either placate Guix by violating the license, or switch my preferred font.
Awful.