There are some more pics in this thread...
http://www.apparelyzed.com/forums/index.php?showtopic=3804Below is the construction of the seat, just cut the bottom off a go kart seat, take the upholstery off the bike seat and cut a piece out of the foam. Drill through the two seats. Put the upholstery back on the bike seat and bolt the two together with 4 long nuts n bolts. Then take the whole thing to a professional upholsterer to have covered and padded, you also get them to put in a couple of velcro straps. Make sure you have a removable cushion covering the bolts as the bolts come lose with time and you will need access to tighten them up.



As far as holding on im not sure, im a para so i have good hand and arm strength. I had trouble balancing when i first got on a quad with no ab muscles but after a while you adapt. All i can suggest is if you can ride a bit just persist, you will get used to it and it will feel more natural the more you do it, just take it easy and build confidence slowly.
Perhaps a different shaped contoured handle bar grip would help you hold on better??