If they're truly flush mount, and glued in place with urethane then where would a drain be located and why would it be necessary? On the 'glass body we had a lip about 1/2" wide, and the guy laid in a rope of urethane that was nearly 1/2" diameter, smooth around the lip and smoothly set the glass in place and pressed it down to seat the bead evenly. Then a tool to take away the squeeze out, and a finger dipped in mineral spirits (I think) to smooth it to a final finish inside and out. No place for water to get in, nothing to drain, nice smooth look. Just my $0.02.