You don’t need a jig to build your deck beams and you can get the exact curvature you want. This may be a little confusing. If you picture your deck beam as it is installed in your boat. The layers of plywood are horizontal. Looking forward in your boat you are looking at the edges of the plywood. What I did was make my deck beam with the plywood layers going vertical. You are then looking at the side of the first piece of plywood when looking forward in your boat. I started with a template in the profile of what I wanted the deck beam to look like (looking forward in the boat). I then took the template and cut out a few pieces of plywood in this shape and glued them together. There is no bending or jig required. You can use any plywood you have no matter how thick it is. You could even make the deck beam out of a solid piece of wood if you wanted to. It can be as wide and/or thick as you want. As I cut mine by hand, the top edge wasn’t perfectly smooth. That is OK as after a little filing I put a layer of thickened epoxy on the top edge before I put the deck on. This filled in any spaces between the deck beam and the deck. Hope this makes life easier for you.