I managed to polish up the extension pieces I made for my bookrack tonight. The design has changed somewhat from my original idea, as it was way over engineered for what it needed to do.
The boot rack now stands on 4 aluminium feet are attached using M5 screws inside the extensions, and the whole lot will be attached to the car using 4 stainless bottle screws that will get cleated to stainless eyes attached to the roll bar and the bumpers:
I modified the rack my removing the end screws holding the outermost cross pieces in and replacing them with M5 stainless eye bolts. The bottle screws hang from these.
I'm particularly happy with the finish I got on the aluminium this time:
The bottle screws have a right hand and left hand M6 thread, so as you turn it either way it either extends or contracts. If they don't quite reach where I want them to go, I have some stainless rod that I will make into longer arms for them.