I hope the thread will be of use. The process I'm going to use is by no means unique - it's just a collection of ideas I've gathered and used over the years. So there are other ways that will work too!

I started by cutting out the parts for the box that will contain the silicone mold material. The basic dimensions are determined by the canopy dimensions with some extra around the sides for the material itself. In this case the canopy measures roughly 13" L X 2-1/2"W X 2-1/2"H. So the box interior dimensions are 15"L X 3-1/4"W X 3"H.

The box parts were cut from scrap out of my lumber pile. The sides are 1/4" utility plywood and the bottom and ends are 1/2". There's no magic in these sizes, but the sides do need to be stiff enough to resist bowing when filled with silicone. The bow itself would not be a problem but it might throw off my calculations for how much silicone I need to mix.

Before I assemble the box I'll fit some pieces of blue foam to fill in some around the canopy so I can reduce the amount of silicone I'll need. The stuff is fairly expensive and there's no need to waste it.