Craft mesh works well. Black zip ties, and you never even know it's not part of the tank.
My betta hasn't had a problem without it, but if yours is, that's probably a good way to go. Make sure your flow is down far enough? Instead of the big sponge over the outflow, I put a tiny piece of sponge inside
the outflow spout (just pop it off, and put a little pice about 2/3 the size of a pea into it) and you can really fine-tune the outflow to wear it's not an issue. Less outflow means less water flow sucking through the intake, so easier for bettas with especially large and flowy tails to get off of it.
Fairly common for bettas to like to sleep there, since it gently holds them in place with zero effort on their part.