As it currently stands, it will hold a bike and 2 wheels (with the loss of the top-box). the bike is clamped at the fork, and also a bungee over the bottom bracket. It slots between the 2 verticals (and can be strapped there as well). The wheels are strapped to each other through the frame, and to the rear luggage rack and the handlebars.
The main platform is secured by a wooden brace that straps to the grab handles, and a second one that is bungeed to the luggage rack. The bottom bracket rest has 3 straps securing it to the luggage rack.
Points of risk I can see are;
1) One of the wooden braces snapping. These are the main items holding them down. I think replacing these with metal would be preferable.
2) Vertical comes loose. These are table legs covered in foam cladding, and are held in place with 4 screws. I'm thinking that to increase strength I could drill holes through the base plate, and screw the table legs on from the other side...that way if a screw fails the table leg will remain in place, held by the baseplate and the saddle underneath.3) Base-plate could be better fitted into the handles. if I made it wider, and more shaped, it could fit far snugger onto the pillion seat, leaving less leeway for movement. It would also improve the surface area of the struts.
4) Base plate can bend with the grain. The way I have it cut, the base plate has the grain of the wood going across the bike...really it should run along the length of it, as this would improve it's stiffness. Maybe a slightly thicker bit of wood as well (it never hurts).So, probably a re-build is in order, though that actually won't take too long, as the core design works, and I have the tools, and measurements.