ed
one car we have has a kirky style seat supported on the sides by plates. the plates are attached to 1" tubing that goes to the drivers side door bar, the tunnel and at the rear to the cage at two points. well supported but I think notto the letter of the rules. at Lime Rock I saw a new IT car this year where the seat was attached to a 1" square tubing frame. That frame was attached to the floor the cage on the drivers side, the rocker on the drivers side, the main hoop of the cage and I think the tunnel on the right. It was a great installation but I am not sure it would stand a protest.

david
good point what is structure. in once sense it may be better for the rule book to be vauge in order to protect the club.