If you have a situation like mine (sole is soft due to balsa rotting away) the preferred method of repair is to go below and cut the lower f/glass panel from beneath the sole and insert new balsa. After that, reinstall the lower panel and 'glass' it in.
I am not going to do that as I don't have the faith I could do it.
My plan is to drill holes into the upper panel and vacuum the old balsa out. Then fill the void with insulating expanding foam. When that sets, I'll use an allen wrench with an electric drill to create a void in the foam at each hole, vac the material out and fill the resulting cylinders with epoxy resin. I should end up with a firm sole supported by a series of resin posts with the the foam adding a little strength but keeping water out and providing some sound deadening.
Wish me luck.