To support the balsa, rather than cutting props to size use a bunch of scrap 2x4 ripped down to about 3/8" or 1/2" thickness and some to 1/4 or 3/8.
Wear a hard hat when you put the balsa in place, then hold it with your head and place one of the strips across then spring a few of the other boards up to hold it in place. Put as many cross bits and springs as you think you need. The hard hat's easier to clean than hair, and frees both hands to speed up the work.
The strip across the balsa evens out the pressure so you don't get divots, and springing the strips in place to hold it works better than trying to cut a brace with an exact fit, and 2x stock is tons cheaper than buying dowels.
You can place them as close together as you want for more pressure if needed.
tape on the cross bits will prevent the resin from sticking if/when it starts to bleed through.
the spring makes it self adjusting so it takes up space as the resin levels out
Much easier to do it from outside.
You DO need two layers of fiberglass for strength. But the fiberglass can be built up in place, use "T" headed pins, or very short staples to hold it up, then remove the pins or staples as soon as the resin cures enough to hold the cloth in place