Update: I cut a big hole in the vertical face of my shower seat and installed an 8" inspection port. Bomar, I think. There's a nice little unused and otherwise inaccessible space under that seat, and by placing the port in the vertical face, it doesn't have to cope with the greater likelihood of leaking that might present if it were installed on the seat surface itself. May not have mattered much, since they seal with an O-ring. And the seal is flawless enough that I store toilet tissue in there, and nary a drop of water has ever intruded. An easy and very effective project.
Dan Morehouse