[boxhead]

although a different hoop design to yours, here is mine.
The main uprights slip inside pipes that are welded to the frame (done like this by kit supplier) then there is a though bolt that secures it.
You can see the round tubes infront of rear springs.

Looking at the far right of picture, you can see a flat plate, this is one I have fitted and welded (not painted at time of picture), I have used this as the base mount for my down leg/stay on roll bar.
This sits flush with underside of boot floor.

And here is a picture of my mount for stay to boot floor.

The flat plate has a tube welded to it, this slips inside the stay and again is secured with a through bolt.