I second the Bondo suggestion, but it isn't epoxy. Its use as wood filler is fairly recent, but it works pretty well in my limited experience. My son used it to do cosmetic repair on the rafter tails on his roof and it has held up. Just don't expect it to perform like epoxy.
The zoo where I used to work uses epoxies to build things like artificial tree trunks, vines, starfish and barnacles. I'll see what they use and how they modify it. I know they use fumed silica to thicken it, sometimes as thick as modeling clay.