I got around to adding the support bar. It's a variation on slaughters wooden bar.
I used an 80cm length of Rhino HD roof rack bar. This bar being extruded aluminium is light and strong and designed to take a variety of fittings. I already had a strap type kayak holder which fits well.
Using a drill press and a 25mm/1" hole saw I drilled right through the bar where the cart posts go. I had to elongate the holes a bit with a file as the posts taper outwards. The bar has an advantage in that it strengthens the cart as well.
The bar with fittings can be slipped over the posts.
The combination of balls and straps supports the hull well.
Eventually I may fit the Hobie trailer cradle supports to the bar similar to sun E sailor's clever mod found here: viewtopic.php?f=73&t=35157
I would cut off the ama supports to narrow the width. I find a strap around the hull and under the ama, supports them well enough.
I prefer the scupper cart for removing the kayak from my trailer (two level) but the disadvantage with this cart is getting it back in under the hull on the water. The long cart posts require deep water to fit and the balloon tyres are very buoyant. Next mod will be removable posts!
PS- Thanks for that suggestion jms2756 but I am loath to add any weight to the wheels. I think it would work though. I used to fit the Trax 2 by removing the wheels. You'd have to be careful of the washers/spacers though with this mod.