Final design, time to start making it now (well in a few weeks anyway). I slimmed the frame down which dropped 40kg from it. I hope to have the whole thing under 500kg excluding tyres. Also added some commercially available windows, ups the cost a little as they are $160 each but they wind out and have fly screens on them as well.
Also made the lower part of the tongue box accessible externally (may end up being a vented battery box) and the top part internal for somewhere to put the keyss and wallet etc.
Cheers
Steve