I have pretty much finished all of my fiberglassing on the my box, and am in the sanding/bondoing stage. The ring I made out of MDF is recessed in the opening. The circle was about 95% perfect when I cut it, but now that I have glassed over top of it, I've been having trouble getting the fiberglassed part even close to having a nice edge/circle. What suggestions do you guys have? I was thinking about using some of that flexible stuff that is used to cover the corners where drywall meets, and then bondoing over it.

Thanks,
Chris