Because 3/4 plywood is not really 3/4 of an inch thick believe I can get two sheets in the void.
The reason for the filler question is two fold. One there are areas in the transom which the factory added extra rosin/fillerthat prevents the plywood from fitting snug. Second there are lots of chip outs, drilled holes, etc which must be filled.
The question concerning the Epoxy Products Co. is that they have a lot of epoxy products for boats and the prices seem to be lower than other companies.