I put a layer of fiberglass on the whole inside of the door, extending about 3″ beyond the front, back, top, and bottom of the door as i was worried about the spring back and overall door strength / durability as the single layer of fiberglass didnt make the sides seem all that stiff vs. the top with 2 layers. To save epoxy, I didnt wet out the section over the window which will be cut out. We’ll see if the slight increase in thickness causes any fit problems, but because it also thickens the area around the door, i think the door seals will fit ok. It wasnt all that fun crawling in from the back several times to position the cloth and wet out with epoxy, but it did stiffen the door up as it now has 2 layers of fiberglass (and 3 where the joint is due to the fiberglass tape) separated by plywood core.
We just cut the doors out 2 weeks ago and didnt notice any spring back to the outside. If anything, they might have sprung inwards slightly, which would improve the seal. So I would recommend it (but note we havent fitted the door seals or hinges yet)