I've got a P1 rig too. I'm looking at the picture and can see how the front is mounted but not the rear. I also have a Sparco Rev II.
Is your seat exactly the same width as the inside of the P1?
Any links to the hardware you used would be appreciated too. Cheers .
My seat is nearly the width of the inside of the p1, in this picture you can see that I just got an piece of angle from the hardware store. It is40 wide but taller and maybe 4mm thick steel.
I just put a bolt down in to the 4040 profile underneath that is taking the weight of the seat and the bracket is just keeping the seat in place. The bracket is just something like this https://www.mitre10.co.nz/shop/carinya-heavy-duty-angle-bracket-40-x-80-x-40mm-galvanised/p/216886
For the rear ones you slide them in the t slot until they touch the side of the seat. The top ones cant do that so you can just put a bunch of washers together to make a spacer.