Instead of carving my own hickory tacks for this banjo, I decided to purchase a bunch of premade, lemonwood cobbler's pegs. They hold the skin down very tightly, and are a more historically-informed choice than the upholstery nails typically used by builders.
I find that shellac works nicely for sealing the inside and outside surfaces of the gourd. It's advisable when shellacking to apply one light coat, let dry, buff with very fine sand paper, and then apply a second coat.