Lead ingots would add ballast, nothing more.
I've never heard of expanding epoxy, usually urethane "insulation" foam is used as an expanding material. And some of that is non-porous and waterproof, others not. With any large volume of epoxy you may also have some issues with heat, it gives off lots of heat while curing.
If anything expands too much and pushes the walls of the "slot" too close to the centerboard
, that may also be a problem.