I can't see the link I'm afraid, but assuming you used craft mesh, you could double up on it. I find that makes the dividers stand straighter and stay more stable. Another option is to silicone the divider in, but thats a little....permanent. Up to you though.
There are those few bettas who just aren't suited for divided tanks, however. Generally most of the fish I've had do alright, but I've had a few that just absolutely hated being in a divided tank. Your boy may just be one that does need to be on his own....even if you get the divider to where he can't get though, he may still stress himself out trying to find a way or just plain may not be too happy.
Then again its also possible he may settle down once you get the divider secure, its hard to say since I don't know the fish, lol. So its your call.
Hope I could help, best of luck!