Norado--If your diagram is to scale--there is not enough taper between the ply and the foam. Make the ply a little wider and the taper more shallow--it will finish better and be stronger.
If you find epoxy a little more expensive, it is possible ti use Vinylester resins instead, but epoxy is my resin of choice for my work on account of its ease of mixing and controllable curing periods depending on which hardener is used. Accurate proportions are essential however.
I use 3mm plywood marine grade, which is quite cheap
in comparison with other thicknesses of plywood, and I use it as a base if I have no mold
or table from which to work. One can get it melamine backed if one wishes to coat it with release compound and peel it away later, or just leave the ordinary bare plywood dress surface to get an apparent timber surface to the ceiling. Make up a frame and do a small panel first to see it the method suits--I have used it to make all sorts of things from cabin
tops to shower
cubicles, using the melamine side to the resin/glass after first coating it carefully with Carnauba wax and release compound. Used carefully it would be possible to pre-form all of your deck panels
in a smaller area, them move them to the boat and bond them to the deck beams. If one was careful one could make all of the panels
and them just cut them to shape and finish the edges later. If you recover the melamine ply instead of leaving it there as part of the structure, the same pieces can sometimes be re-used for other purposes.
Anyway, I fit my plywood in place and lay my first medium to heavy glass covering over it. This is the mat that will be in tension when the deck is loaded. Once that has set, the ply will be far more rigid than it was. Then another good coat of slow-set resin is rolled on and the first foam panel is cut to shape and laid in place and weighted down. I find bags of washed gravel work well here--they stay in place, are not to heavy and do not provide any point loads. Once you have the panel n that and weighted down, you can roll out the excess resin from beneath lifting bags and replacing them as required. Roll not too heavily. just firmly, from one side to the other to expel all air. Do not rush the job, take it a sheet of foam at a time, once the foam is glued to the glass it gets a little bit stiffer again.
If you use slow set resin you can stop the job at any time and mop up any excess resin. If you use a medium speed hardener you will have less time to work the job, and the gain in pot life is an advantage too good to pass up.
When you come to the top layers of glass--I use peel ply cloth to save me having too much messing about afterwards. Two layers is a good surface if the glass is heavy enough. If you use heavy glass mat, a slow set resin is essential to be able to get it properly saturated. Peel ply is cheap
and saves a lot of work. Only a few spots will need the grinder, and the surface will take a good epoxy undercoat.
Once that ir ripped off after the resin has cured, your deck should be extremely strong