Randy, that depends on several factors. Personally, I'd use something more substantial. The reason I say this is simple. If the humidity is high, or you're in a light rain, you'll arc over the best of mounts even with 100 watts.
You can make an insulator easy enough if you have access to a drill press. I found 3/8x24 inserts at Ace hardware for $2 each. They are threaded 5/8x 16 on the outside as I recall. Drill the right sized hole, and you can turn them in without using a thread die first. Just screw a 3/8x24 bolt with a jam nut into the insert. With a little effort, you're in like Flynn. The inserts can be pinned to keep them from turning back out. The two inch Delrin rod can be purchased from a variety of sources.
Or if you can find one of the old GE mounts, they'll handle 100 watts through the coupler no problem.