<<I've wandered West Marine and found many hatches that appear that they
may work but I'd like to hear from others before I jump in.>>

The problem here is the Marine Ships stores are selling just the
replacement hatch, not the hole, or the gasket.

They are some that are a flatter, lower profile than the Westy skylight
for sure.

But how do you propose to keep the water  away from the hatch without a
preformed drain / gutter?

Just use a gasket?

Watch out for the rain, and car washes.