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. . . So the last layer be just slightly larger than the transom with consecutively larger layers under that, 4 layers sound sufficient or should I beef it up and go with 5? Should there be any kind of mat or something between the layers or just need 1708?
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4 layers is plenty. You might want to recheck Carla's post & photos, but I believe first layer went about 2' around onto hull; successive layers were about 6" less. The main thing is to achieve a gradually tapered thickness to avoid a sudden thick/thin transition. A sudden change in stiffness can create a hinge effect and a stress concentration. For total thickness buildup, I believe you can figure on roughly 1/16" for each layer of 1708, so you may have to trim the stuff you cut out for what will no doubt be a thicker new transom.
A layer of 1708 on the old transom skin (rough it up first) is probably a good idea to insure good bond between the original polyester and the new epoxy. Be sure to wash off the amine blush from any cured epoxy before putting anything over it. Most folks seem to use put a layer of glass/1708 between plies in the transom, but I'd check the Coosa and/or West System web sites to see what they recommend. The Coosa Skip used was about 2" thick so only used 1 piece. When you apply the cabosil before installing core, use a tile trowel to create a series of v-shaped grooves which will allow the paste to spreadout and hopefully achieve uniform thickness. It's also good to install some randomly placed (to avoid a tear-along-the-dotted-line scenario) ~1/4" "vent" holes to allow any trapped air to escape between core and transom skin. Good Luck! Denny