I just did the same thing, how i did it was to use the rear retaining ring that was on the back of the guage as a mold
, covered it with wax and layed it on a cookie sheet with wax paper underneath. Use a very thick layer of wax to seal the bottom of the ring to the wax paper. Precut 6oz cloth to fit inside the ring, mix the epoxy
, and start layering until the thickness is where you need it. Once cured, sand the outer edge to fit with about a 1/32 gap all around, then mix epoxy with colloidal silica to peanut butter consistency. Have some round toothpicks handy, too. Put the thickened epoxy around the edge of the plug
and around the inside of the hole, let it just start to kick, and place the plug
in the hole. use the toothpicks like wedges front and back to hold it in place. Smear some around the back edge with your finger, and smooth the front the same way. After it sets good (next day) sand flush to the bulkhead and fair, sand again, finish out. Mine came out really nice (did 3 next to the companionway) and strong, tested them with a deadblow hammer, no issues. My bulkhead there is about 1/4" thick and does not flex, if yours is thinner some glass mat behind it for support would be a good idea. Good luck!