What to Use to Seal a Hydrocal Plug?

I am making a fiberglass hatbox style hatch cover for an RC sailboat. The original was a rather thick vacuum molded styrene part that had almost straight sides and was way too tight.

Over time the styrene shrank and it split when the owner forced the cover over the fiberglass lip on the deck. I patched the split in the cover and filled it with Hydrocal to make the plug.

After it cured, I used Bondo to add some relief to the sides. I could make the part using vacuum forming but that would require building a vacuum box and a frame to hold the plastic while heating, which is too much trouble for a one-off part. Then there is the original problem of shrinking and splitting.

If I just lay glass over the plug with veil as the top layer, after sanding and painting it should look and work fine, with minimal long term shrinkage.

But what should I use to seal the gypsum cement/Bondo patched plug??? I have shellac, Frekote, Partall wax and PVA available.