I had some filament without spool and needed one that can be quickly done. Thus I decided to create a very minimalistic one myself. You just need a few M3x16-mm-screws, washers and nuts. The inner tube sits strong in the ring, if not, you can glue the two rings.
The spool is not super strong, but it doesn't need to. I Just printed it with 35% infill and 0.4 mm walls with PLA/PHA.If anything breaks I will do it again with stronger walls and more infill. But it seems it's quite good already.
It fits on the original UMO spool holder, but it's larger than the normal spools and keeps the filament a lot less bent.