My TC is all fiberglass, so is almost a boat, but I have worked on it making repairs using fibers and resin.
You can use plywood, just no reason for PT wood even if it were not toxic. PT last a little longer, but not significant.
Coat your entire plywood floor with resin (can get a home depot cheap) and some fiberglass fabric. Attach it to the TC then fiberglass and epoxy over the seam so water running down the side stays outside.
This will be very durable and waterproof.
The Homedepot resin is resin, not epoxy resin, so is a lot cheaper. not as strong as epoxy resin, but for what you are doing you do not need ultimate strength.
Good luck finding a respirator mask during this "outbreak" frenzy, but MAKE SURE YOU USE A PROPER MASK, the fumes will shorten your life.
I have heard people using cotton cloth or whatever they have on hand, like a fabric drop cloth. All you are doing for the surface of the wood is coating it, so fabric should be fine, or even just the resin, but for the seam you will need fiberglass cloth.
Use 2 or 3 layers depending how much strength you need.
It is amazingly strong.
So, if you have already done a repair, then just apply the resin and cloth over what you have done to make it waterproof. you can go right over the paint. Work with small batches as it will cure fast. Using a lot of harnder makes it cure quick and dry hard, less hardner will make it cure slower, giving you more time, but will not dry as hard, or may never fully cure. But for just waterproofing you will be hard pressed to mess up.
You will need to compress the fiberglass/resin composite to get air bubbles out and squeeze out as much resin as you can from the cloth. I used non-stick aluminum foil over the fibered seams and pressed. The aluminum foil peeled off easily for the most part, just a few little bits were left stuck, but under the TC who cares….
You will be working upsidedown, so nothing will be easy.
CRITICAL that temps are 70 or above. high 60's might be OK. I did work and made a tent and used a heater to help keep it warm in winter. Read the label for specific working temp
