What do you mean by the top part of the divider? I am getting ready to divide a 10 gal for my bettas, so I'm taking all the advice I can get. I'm planing on using the DIY dividers, and all i was aware of was the vertical pieces, nothing on top.
He might be a bit sore inside from having the fin hung up in there, and may need a little time to heal. Just offer him something soft ever so often and see if he starts to eat in a day or so. Glad he's still hanging in there. Keep us posted.