You are trying WAY too hard bro.
It sounds complicated, but believe me it is not:
Take that entire plastic tray out. Once out, take some of the drawer liner that you use in kitchen drawers or toolbox drawers (1/16-1/8" thick foamy-rubber stuff). Lay that in the bottom and around the sides. Now use some heavy duty aluminum foil and line the area you want to be your "storage area" ensuring you use the foil to hold the drawer liner in place. If you want to put a lock or hasp, you need to add a reinforcement for it to lock to. I have used a piece of metal plate, others have used wood; whatever floats your boat. Wrap whatever it is in 2 layers of the drawer liner and put it in place. Ensure this goes UNDER the aluminum foil so it makes room for it to be mounted in the fiberglass. Now use some newspaper and masking tape to mask off the surrounding area really well. There is a spray on fiberglass resin (can't remember the name of it) and spray the aluminum foil with it. Let it dry and apply 3 coats. You just made a mold.
Now, remove the masking and then the mold. You can now use the resin reinforced aluminum foil to make an insert. BE CAREFUL! The foil/resin mold is a bit brittle without fiberglass matting to reinforce it.
Get enough fiberglass matting and 2 part resin to cover the entire mold with 2 layers. Do some dry fitting of your matting pieces so you are prepared. Spray the aluminum foil with some Pam or other cooking spray (enough to coat it, but not too much! Too little and the product may stick to the aluminum foil), prep your matting with the epoxy resin, and lay it over the mold. While the matting and resin is still wet, place your lock/hasp support in place and wrap the wet matting in it. This will ensure it is encompassed inside the fiberglass and will be stronger. Let it dry. Apply a 2nd coating of fiberglass matting and resin covering the entire outside of the first layer. Let it dry.
I recommend letting it dry at least 24 hrs. Then, flip it over and and peel out the foil mold. Now you can apply another coat or 2 of matting and resin to the i side if you so choose for more strength. Once its all dry you start fitting it. This requires some patience as you will need to trim and fit and trim and sand and fit and sand until it fits in snugly and how you want it. Once it is fitted, you can line it with automotive/marine carpeting, foam, etc and prep it for your "projectile emitting device(s)". Some people use bondo or wood filler to smooth it out. Up to you.
You can make a lid and attach it with a piano hinge, or use the package tray as a lid. Up to you as to how to do the lock/hasp.
In all honesty, allot yourself a week to get this done. Maybe longer. I would make some smaller molds of simple stuff to boost your confidence and ablilities. Smaller projects are actually more difficult, so don't get discouraged. Plenty of youtube vids out there to provide better instruction. Probably should have just linked one, but since I already wrota all this; you can suffer through reading it!