In an emergency, as the silicone used was leaking and the unit was in use, I peeled off the silicone and used Liquid Nails. It sealed fast enough to block the active leak and has held ever since.
Since then I've used it on other units and it always works. From Home Depot.
I used it just on the top side, not inside the cap at all.
Member AGA, HAS, NASH
110G, 10 adult discus, barebottom with plants on wood
58G, 3 adult discus, barebottom with plants on wood
59G thickly planted tanks, rummey nose tetras, kuhlies
300G goldfish pond outside