When I mentioned small fish earlier, I meant fish the size of a guppy, neon, etc. For a betta I would personally prefer clove oil. When I use the clove oil, I add enough to a tupperware container to knock the fish out (and probably finish the job), then I put a lid on the container and leave it in the freezer until it freezes solid. It's not going to survive that.
I'm sorry about your fish. I've never had a fish freak out in the clove oil. They have all seemed to go peacefully that way. The largest fish I have used this method for is a 6" firemouth that just got too stressed while I was moving to recover.