I do think you can do this from inside the cabin and preserve the deck
1. working overhead is crummy
2. working with balsa overhead is a nightmare
step 1: Cut away all as much wood as possible and get some version of core-cell. cut it to the size of the cut away and shave off the edges, if necessary, to get a tight fit.
step 2: clean the deck (at least this area) and tape off the winch holes from above
step 3: clean the down side of the deck (cabin top) and give a decent sanding
with 80 grit to aid in the adhesion.
NOTE: BE CAREFUL and let the sander do the work... it will be easy to burn thru to the geck
step 4: bleach bleach bleach bleach and let dry! (this is the exact place mold
loves to grow)
step 5: epoxy the foam in place with 105 / 209 (slow)
NOTE: use something like this to keep consistent pressure applied while the epoxy cures
step 6: read a book or drink a 12 pack or re-tile the bathroom in your house, or take a road trip or do WHATEVER keeps you busy for 48 hours and prevents you from rushing to the next step
step 7: re-consider step 6
step 8: lay up fiberglass over the foam with 105/205 (fast)
NOTE: it is messy working over head
and there will b a lot of dripping
TIP: cut a piece of 1/4" wood 150% of the size of the foam AND a piece of 6 mil plastic the same size. put the plastic on the wood, the resin on the plastic, the fiberglass on the resin and more resin on the the fiberglass THEN brush on some resin on overhead foam paying attention to the wood / foam seam. lift
the wood overhead and use the joist jack to press the wood firmly into place. the resin will not stick to the plastic resulting in a well adhered 1st layer of glass.
step 9: light sand with 80 grit and repeat step 8 (2 or 3 times)
step 10: drill and re-install the winch with sealed deck holes and a backing plate
step 11: do whatever you are gonna do for the headliner