Why not do the bottom in spar varnish as well? Not like you have a ton of area. Plus, it's shinier.
Protip: make sure your epoxy is fully cured before varnishing. Wash it with a 3M pad, hot water and a mild detergent to get any amine blush off the surface. If you have amine blush and it mixes with varnish, it turns into a non-curing, non-drying sticky goo. Slow cure epoxies blush more than fast cure epoxies (fast cure locks up the amine; slow cure it migrates to the surface during curing).
The wash with the 3m pad (or its dollar store equivalent) also gives the epoxy tiny scratches for the varnish to bond to.
Warm the epoxy slightly in the microwave so it's thinner. Easier to mix and penetrates better. Warning: warmed epoxy cures faster in the cup, too! The epoxy I use gets a 6sec nuke for 1oz, 12sec for 3-4oz and 30sec for 16oz.