Hi Pyro,
I got myself a windsurfing trailer a couple of years back. It's mainly for road trips after I realised piggybacking 3 boards on the roof and loading 3 kids, plus all my sails, booms ,masts etc. into the van wasn't a good idea

I bought an 8 x 5 galv box trailer from Papas and designed a custom made aluminium canopy to fit on top. I got the trailer canopy fabbed at ACS in Midland (
www.acsfabrication.com.au/). They did a fantastic job, key lockable doors front and rear, gas struts on the tailgate etc.
It wasn't cheap, but I'm rapt with the result. It has four interior racks and I can fit four boards plus all the rig components.
For trips away I even stow the kids bikes and other luggage in there.
You can PM me for more info. I probably have my (hand drawn) plans around somewhere.