Pretty sure you are correct about the 15mm UW Nikkor that it's optimized for underwater...
I'd take a look on eBay and hope you find one unbundled from one of the several 15mm versions and doing a quick search out of curiosity, there are several Voiglander and Ziess besides the UW Nikkor for sales from around $100.
An alternative would be to buy a combo lens/viewfinder and sell the lens. Otherwise, you may be able to hack one from mounting board and duct tape like I once used to do, akin to a sports finder. It may look ugly but does the job!