So I have a decent bike pannier bag with cheap, crappy zip pulls that all snapped off, whilst the actual zipper bodies were fine and stayed intact..

I couldn't find any suitable replacement zip pulls to print here, so I made my own. It is heavily inspired by https://www.thingiverse.com/thing:2594163 but this was re-designed from scratch rather than tweaking that file. The other thing takes a square-ended paperclip, whereas this uses a UK style paperclip where both ends are rounded. The ones I had to hand were about 28mm x 8mm, but you can probably just print it scaled for other sizes.

Put the paperclip on your zipper, then Insert the double-looped end into the slot until it clicks twice.