Installing beams in the chassis is not possible. It's glued together and modifications are not allowed.
BTW.. Almost all damage to the chassis means it's a write-off. No repair work is allowed to be done on the chassis itself.
It's easy to write off an Elise by hitting a curb with the front wheel and tearing a wishbone pickup point on the chassis.
The options here are (I assume a 6-point, so 2 crotch straps.. works better for boys

), depending on what the manufacturer of the belt allows:
- Pull forward and bolt to the front chassis beam
- Pull backward and bolt to the rear chassis beam
- Attach to the seat rails themselves either directly or using a steel crossbeam bolted in place. (mostly used if the sliding seat rails are replaced by bolt-in rails, which already have holes)
- Attach to the seat shell
The seat bottom is only a few mm from the floor surface, so if you want to keep the seat movable (in case you keep the original sliders) then you can't really put much underneat the seat.
Usless trivia: small pebbles or bits of rock stuck between the seat and the floor make the floor resonate like mad.. Everyone gets that one while driving as small stones are thrown into the car by other cars. Scares you silluy the first time
Bye, Arno.