Err, routing a 1/4" lip from your 1/2" okoume is certainly doable, too, of course. I might have better uses for that wood, though!
The 3/4" space will be fine.

Anyway, here's a pic, that I think I may not have ever posted, of a coaming I built from non-marine plywood. It had pretty good grain - better than any okoume I've used. The copper inlay was fun to add, too, but still, a spray skirt completely covers it.