Seat rail mounting.

[mark]

If you are tall and want/need to be low in the car then flat bar, I used 6mm and once the runners are bolted through the bar and floor its rock solid

Box section would be fine if you aint that tall and need a bit of help seeing over the scuttle!

Dont forget the thickness of the seat base when allowing for your final seat height position, mines on 6mm bar, lowest profile runners I could find and a fibreglass seat, this adds up to a couple of inches with the seat thickness taken into account