erazz

There are a couple of ways you could go about this.

Tape/Monokote. Very simple but the edges will be transferred to your mould and would have to be sanded and polished out.

Another way is to seal the plug with fiberglass. This is the more professional approach as it would create a durable plug and no seams. However it requires a lot more work to lay up the fiberglass and then sand, paint and seal the plug to the required standards.

You can also seal the plug with autobody filler. The stuff sands easily enough and can be painted on. You would have to be very careful not to damage the plug as it would be very delicate.