TH,
You may already know this, but just in case......
I have laid up several flat panels using the following method.
Get flat pieces of plywood and sit in on a solid base. A concrete floor will do just fine. Cover the ply with plastic, stretching it tight so there are no wrinkles.
It takes about 6 layers of 1808 fiberglass cloth ( bi axial with a matt backing ) to produce a 1/4 inch thick panel . Wet and roll out the fiberglass on the plastic.
Now cover this with another piece of plastic and then put another piece of flat plywood on top. Make a sandwich.
Weight down the top piece of plywood using cement block, tool boxes, batteries, etc.......
There will be a lot of squeeze out of resin so make sure all the pieces of plastic are large to catch it.
The next day you will have a solid flat fiberglass panel. Cut it to shape on a table saw or with a skill saw, sand it prime it, paint it .
Quick and easy.