Whale Urchin thru deck mount seems to work and is pretty common on ~30 ft new boats. most WM don't stock the thru deck, and have to order it from "the Warehouse"
but they do carry the repair kits same as for the surface mount. I used the thru deck on 505 and 526. has a pretty plastic cover but kind of flimsy.
existing hole was ~ok ... have to redrill mounting holes and fill old bolt holes to position it ok.
