Help with hardware removal on tube

I wasn't sure what folder to post this in, so feel free to move it to wherever it belongs.

One of my chins' tubes started slipping on the front side, so I kept tightening it. I guess because the bolt is near the foldover, I just couldn't get it tight enough to keep it level. Now it's stuck, and it won't budge! I have a lava ledge underneath the tube to support it and keep it level at the moment. You probably already know, the bolt is flat (no indentation for a screwdriver). I tried gripping the edges with pliers, but that didn't work out either. Maybe I need to try a different kind of pliers? It's been a few weeks since I've changed the fleece on it. Thankfully, neither of the girls urinate in it, and I was able to de-furr it.

Any ideas on how to get it off?

Thanks!
Trish