I’m assuming that you are referring to the old style fcs x2 plugs (2 plugs per regular fin). If that’s the case, they are normally installed after hotcoat anyways. Installing 2 on a finished board will be the standard procedure of drilling hole, cutting h-pattern to connect to deck, filling with resin/milled glass, set in plug, sand smooth once cured.
The h-pattern is when you drill the hole the depth of the plug with a spade/forstner type bit. Then you take a hole saw style bit and finish drilling the outer edge of the hole all the way to the underside of glass of the deck. It creates a column that attaches all the way from the bottom to the deck to give it more strength. The milled glass i use is almost like powdered fiberglass. I’m sure chopped would be fine as well. Here is a cross section of the h-pattern.
Milled Fiber. 1/32" ( I think?). Can be bought at most supply shops. I put a dab of white pigment in mine. I like to use one plug and install the little single tab fins like Rainbow and True Ames makes. I’ll post up a pic of a single plug install later.