I bought a old long narrow quad bike trailer and enclosed it for windsurfing. I used timber I might regret this at some point but I have already taken it to Perth and back so should be all good I have used Harken hardware and stainless steel patch fittings so no expense spared there Once its finished it should be totally awesome I'm hoping
Has it got brakes? Reckon you will be over your 750kg there once loaded....
Also be mindful that too much addition of length at the rear will make it want to tailwag so you will want good ball weight (downwards at front) and not tow it with a hatchback (tail wagging the dog analogy, not fun)
I might regret this at some point 
but I have already taken it to Perth and back so should be all good
