Its pretty simple.
1- Cut 1/8" plywood to the size of the panel you want. 2 pieces for both sides. It would be ideal to use marine grade, but it is hard to find and pricey. You can use anything, even cheap floor underlayment since you will be coating all surfaces with epoxy. Make the wood slightly oversized, you can trim it down later. Don't try to be too perfect at this stage. If the panels are over 4X8 you can butt join the panels, they will be covered in glass closing that seam.
2- Lay one plywood side on the floor, cut the foam- the stuff you can get at Home Depot or Lowes like in the picture. I put wood support members about every 2 feet. I used poplar because it is light and I have lots of it, but you could use cut down 2X4s. Position the wood and foam like you want it, frame in widow opening if you want. If there will be anything attached to the panel put a wood structure in to back it. If there is a joint in the plywood make sure there is a wood member to glue the plywood to. Glue the top plywood piece to the wood frames. I use tight bond III and a brad gun to glue. it down.
3- When the glue dries carefully turn the panel over and remove the foam. Coat all the inside surfaces with epoxy, put the foam back in and glue the top plywood layer on. I would size it and make sure the panel was good before I applied fiberglass the outside. The panels are very light and rigid.
I'm sure this sounds much harder than it is. Wood/Epoxy is a very forgiving material to work with.