Transoms outer skin has been removed

Took two half hours with a 4 1/2 " angle grinder to bevel and grind through the outer skin. Then only 30 minutes to remove the skin from the rotted mush. WOW the dust is something else. Have some pictures of the project. Will post as soon as I read the"How To" severial times.
Just for information the gel coat and fiberglass on a 1978, V20 Cutty Cabin is right at 7/16 of an inch thick. would have never guessed it would be that thick. That thickness will be a great base for rebuilding the transom.
In your collective and learned openion which is the best epoxy filler for me to use along with one 3/4 inch and one 1/2 inch plywood between the inner and outer skin of the transom? Also, what do any of you know about www.epoxyproducts.com?