Sorry to hear that putting your shrimps with the betta didn't go well. My first betta couldn't have shrimps either as he would go after them. My current betta is ok though, he gets very close and watches them but nothing else?
It would be best to have a filter if you can get one small enough, and if it is adjustable have it on low flow because it is a small tank, the shrimps are fine in flowing slow water,! Are you planning to keep them in this tank long-term or is this tank temporary until you decide where to go next with them? It is possible that some may die from injuries/stress/shock etc, but hopefully they will be ok.
On Saturday I wanted 8 ghost shrimp for my betta (he's in a 12 gallon). Mostly for extra cleaner, while also expecting the my betta trying to eat them. After one night, he ate out the eyeballs of 3 shrimp while the remaining 5 seemed to be attacked but not too damaging. Currently they're in a 2.5 gallon tank, no filter, gravel as substrate, 1 moss ball, 2 plastic plants, and a small castle for hiding. For food I put one algae wafer (in tank right now) and plan to put pieces of lettuce or cucumber after a couple of days.
My question is, do ghost shrimp need filter? I currently have 5 of them in the 2.5 gallon tank. If they do I'm planning on to get a certain sponge filter that's safe for shrimp and small enough to not cause too much current.