So the steps I've seen are:
drill a hole bigger than you need 1/2 inch or so
fill with epoxy and let it set
drill out a 3/8 inch hole centered in the epoxy circle
use 3/8 inch outer diameter plastic tubing with a cord or coax cable through it as the form for the fitting make sure you leave enough tubing hanging onto the deck so you can yank on it
now here's where i see a few different things.
Use thickened epoxy to make a mold around the tube
OR
use fiberglass sleeves around the tube and epoxy
OR
make a mold around the tube with fiberglass cloth and epoxy
or
some combination of above.

I imagine a bit of graphite powder to make all the epoxy black would look pretty sweet.
I also think you will need to slightly round the edges of the holes to prevent fraying of your deck lines.

Once the epoxy is hard, pull the plastic tubing out and poof fitting done.