I built a Coho and the best filler color match that I found consisted of 2 parts wood flour and one part glass microballoons mixed with epoxy. I made my own wood four by sanding some of the plywood supplied with the kit.

I think if you bevel the edges of your panels, the panels will not fit as well. The seams are part of the look of the boat. Try to get them as even in width as possible and they will look fine. Don't over tighten the wires.