Mark, if you are still looking for a way to pour a trebble in the middle of a big grub I can help. I make very large tails for Musky lures that have a 7/0 treble molded dead center.
The first thing I do is make my form out of wood. When I have it finished I cut the form at the place where I want the bend of the hook to be. Then I drill a hole big enough for the eye end of the hook to pass thru. Carefully mark where the the round bend of the hook will ride and using a round file make slots in the wood. When the slots are deep enough the cut off piece of the mold should fit back on the other half of the mold flush. Fill the area around the hook shaft with plastic wood. Glue the two halves back together. I dip the whole thing in a wood sealer until the surface is smooth.
Now you need to make a shallow box to be the pan for pouring the rtv (blue works good). Place one of the trebble points straight down and mark the box where it rides. Cut a tight slot for the hook point to slip into. You now have to also have to cut a small slot for the hook eye to enter. When you are done the mold should be laying flat on the bottom of the box. Depending on how much rtv you pour will determine how much of the other two points of the hook are covered. If there is rtv covering some of the hook on top, just carefully use a razor blade and slit it so the mold comes out without tearing.
I have molds I have been using many times and they still are in good shape. Be sure to tape the slots in the wood box before pouring plastic. Hope that gives you some ideas.
Ed