Fastening Pedestal Seats in '74 Tsunami?

[Handful in NC]

What's the best way to fasten new pedestal seats securely to the deck on a Sceptre/Tsunami? I'd rather avoid toggle bolts or screws.

I was planning to put an access hole in the center of the pedestal circle, large enough to reach through to place T nuts on the underside, but it looks like I'd be going into a foam filled box.

This image was pulled from another Forum member, and I assume is the same construction as my boat.