drops from the cage to the floor, making something of a U or a box, depending on your access. use square tube. stitch weld this to the floor. drill holes down through the "cage" and floor where you want the seat mount bolts (assuming standard FIA seat style side brackets), drill back up with a larger hole only through the floor and lower face of the square tube. larger hole should be large enough to allow access via a hex head tot he 8mm or whatever size socket head screw your use to bolt through the seat mounts. locknut on top.

OR you can get a seat mounting adapter from "planted" who sell them for a BUNCH of cars. bolts to the OEM seat mounting points which are usually pretty strong, and allows bolting of standard FIA side mounts or direct bolting of lower mounts to it. easy way to add a seat to a car that is already built / for those with less fab time / access / whatever.