I love these things: https://northwater.com/collections/on-board-storage-systems/products/loop-track-anchor

Do you think it'd be possible to make these with some fiberglass/epoxy and webbing? Does anyone have any suggestions on the best way to go about this? It'd need to be strong enough to keep water bags secure.

If you want a secure attachment and also 'soft' webbing, probably buying those would be the easiest.

You can saturate webbing with epoxy and stick it into a boat, but it's fussy to get the loops where you want them. Of course, this makes the webbing hard.

You can make up some glass padeyes fairly easily - I use them for lashing points inside my kayaks.
I used to just 'stick' them in with thickened epoxy, but now I lay glass tape strips over to make sure they won't pop off.

A simple mold with a board, dowel or drill rod, red tape to cover, some wax. Lay up some scraps and press down the edges with tape-covered sticks. Pop off the mold when cured, trim the edges. Cut into sections.
It's better not to use your best bandsaw blade for this kind of work. :-)