Vacuum bagging!
If you are going to attempt the "under-side" repair, you might want to look into vacuum bagging.
West-System has a $3 manual on the basics of vaccuum bagging, which might be worth looking at.
The basic principle is that you put in your epoxy, stick on the core, cover the whole thing with some polyethylene sheet sealed at the edges, and draw a vaccuum on the inside. That way air pressure (~14.7 psia at sea level) pins the core to the ceiling evenly. And presumably better then trying to hold it in place with braces, etc. With a really good vaccuum pump, you can probably get almost all of that 14.7 psid, but even a moderate one will let you get at least 10 psid. For the whole underside of the forepeak, that would add up to.....~2000 square inches, and ~ 10 lbs per sq-inch diffential pressure (psid), maybe 20,000 lbs of clamping force evenly over the whole area. Sure to squeeze out any excess air bubbles and excess epoxy. And since you can vary the vacuum, if you feel like you are going to squish the core, you can reduce the vaccum, say to 5 psid.
Ebay can hook you up with a vaccuum pump for ~$75.
When I replace #3's core, I plan on first trying the vaccuum bag technique.
Some numbers to remember:
29 inches Hg vaccuum ~=~ 14.7 psid
is almost a perfect vaccuum.
10 psid ~=~ 1440 lbs clamping force per square foot.
Let me know how it goes!
-km
aka, "sell out"
S/V Beyond the Sea
C&C 35 mkIII