Tiger kit car seat mounting

[cps13]

Hi

I have bought these seats (or very similar)

http://www.ebay.co.uk/itm/New-GRP-Kawasaki-Green-B...

and these runners

http://www.ebay.co.uk/itm/DOUBLE-SEAT-SLIDER-RUNNE...

The seats have 4 holes in the underneath into the fiberglass, I can screw the bolts into these but it is obviously not that strong/safe. Looking through the Westy build manual I have I cannot find anything on seat mounting.

Do people usually put rivnuts in the base of the seats?

Cheers,