I'm new to the world of DIY BDSM, but not to DIY or carpentry in general. I've decided that my first build will be a bench for the bedroom that will look like a casual piece of furniture, but when the hinged seat is lifted up it will have storage on one side (for toys/restraints/etc.), and a queening seat on the other side. The queening seat will have a cut-out on the seat, and will be essentially a hollow box underneath, and the front of the hollow box will have a cut-out for a head to enter, with a slot for a blanking plate to hide the cut-out when not in use.

My main issue at the moment is how to waterproof the inside of the queening seat box. The seat will be used for oral play, but also watersports, and since it will be made of wood, I'd like to make it easy to clean. I've thought about tanking the inside of the box with some kind of tanking, and I've thought about trying to find a plastic tray or box just the right size so that it can just be lifted out and emptied after play, but I'm still not sure about the best way to go about it. Has anyone done anything similar, or any ideas for how else I could approach it?