Best way to cut the arch in a transom is?

[Beach]

Best way to cut the arch in a transom is? We have rebuilt the transom in my sons SC20, decided to go with a full transom, Bracket and Lenco tabs. We are locked into a 20" motor and are here on the choppy Chesapeake bay so we decided it would be better to close in the transom completely and run the 20" Motor, bracket and tabs combination than to allow lots of water to come in due to the low transom and heavier motor. So how does everyone cut a transom arch? I used a long piece of ply ripped 2" wide the other night and got a pretty nice line/arch to cut by but by the time I was finished with a jig saw the finished product looked like I had been drinking HEAVILY...so I simply glued the cut off portion of coosa board back on with some thickened epoxy and will have to start over again...

I'm using a good jig saw and good blades, although the coose dulls even a good blade pretty quickly. I ended up with an uneven angled cut on one side and a flatter cut on the other...any ideas?