LongTimeAgo, hows it coming along?
I figured I would just add to your thread instead of starting anew, luckily it's not "old news" yet.
I'm just at the fun part with mine, racks, electrics etc.
It started with a well used, but solid 7x4 box then added the extension down the drawbar.
I decided to make the frame removable, although it will take a little effort to do so.
Should have taken more pics during build, but here she is. Maximum board length of 280cm.
I used composite panel, 3mm of plastic with a very thin sheet of aluminium on each side, i bought some as seconds and saved a bundle. This stuff is so easy to work with! Sikaflex and Rivets hold the panels on...hopefully.
The internal racks hang from the roof, allowing me room for other holiday gear or a swag.
Plans include a 12v system for a water tank and pump to wash gear off.
Air vents, lights, small item storage and so on.
Pretty chuffed with the final product, certainly makes life easier not having to load and unload the car every trip.

Any ideas are very welcome.
Cheers