3M makes a product, Scotch 2200 mastic pads that is excellent for that situation. I literally removed my two antennae from my SM two days ago when I took the masts down. I intend to use it when I put them back up. Scotch 2200 is a very long lasting self-sealing waterproof product that comes in either tape form or pads (roughly 3”x5”) that can be cut to size.
S/V Celtic Cross