I obtained the belts from a local car accessory store, they are called Le Mans, that's all I can remember about them. They are three point as you will see, one fixing (clip) at the back and one each side. I removed the rather prominent makers labels too. They are fairly hefty woven type belts, not the shiny nylon stuff.
The belts are fixed belts, i.e. not inertia reel type; I can reach everything with the belts tight enough to stop me hitting anything. The hand brake is the furthest away but mine is adjusted to be within reach.
The photos show the three different fixings
'Fix 1' and 'Fix 2' show the eye bolt fixing through the bottom of the wheel arch, the tapped plate supplied with the belts is fixed over a 1/8" plate to help spread the load.
'Fix 3 and 4' show a steel strap from the chassis to just below tonneau board, fixing'c' is to the chassis 'b' is to the shock bracket and the top has a short steel angle 'a' which holds the tapped plate supplied with the belts, this coincides with a hole in the tonneau board and receives the eye bolt for the rear fixing,(close fitting to the board was essential here, for the eye bolts supplied were not too long, on modern cars they don't expect to go through plywood!) If you get longer eye bolts make sure they are welded loops or get them welded.
'Fix 5 and 6' show the rear eye bolt fixing and the slots in the tonneau cover. You will see that the steel strip takes up a raking angle to miss the sidescreen box and this rake is approximately in line with the seat belt angle from the tonneau, therefore the potential stress is straight along the strip.
'7 'f' shows the left hand fixing to the steel strap below the rear of the tunnel (the right hand is similar. Again, note the tapped plates, they they are mounted on the topside of the strap, I would have preferred them to be beneath the cross strap but the eyebolts supplied were not long enough; however, it seems solid enough as any upward pull is transmitted along the strap and tends to put a downward force on the other side, so both sides tend to work together.
SB 1 and 2 may help to clarify the process.
There are also a couple of interior photos which show the belts and the tonneau cover arrangement.