I've got a cylindrical cavity about 1.25" in diameter and 1.5"
deep that I plan to plug with a cylinder of styrofoam that's
slightly smaller in diameter.

West System resin and 206 hardener plus a little 403 adhesive
filler and/or some chopped-up fiberglass.

Smear the inside of the cavity with the mix, making sure there's
enough at the bottom to smooth over any irregularities, smear
the plug, pour some of the mix in to the cavity, moosh the plug
down until the mix overflows the edges and the plug won't go down
any further, and let it harden - trimming jelled resin as
possible/convenient.

My concern is that in that confined space, the resin will go off
too hot and melt the plug and/or the surrounding styrofoam.

Is this a legitimate concern? If so, how far can I depart from
5:1 and still be sure the stuff will harden?
--
PeteCresswell