good question - I wanted such a thing, and I've ended up with a folding launch trolley rated at 100kg to go in the box trailer with the deflated dinghy&outboard (just the dinghy which deflates, not the outbard!) to take abroad, and a proper indespension trailer for inflatabes rated at 350kg for taking it to the water in this country without having to deflate it.
My rig is 360cm with an 18hp and weighs 110kg without fuel, etc.
trailers nowadays (if maunfactured after a certain date [1982] - see towsure.co.uk/ntta/law/law/identification_plates.asp) must have their rating plate visible, so homemade ones become a bit difficult to make legal.
a few of the manufacturers have good web sites (indespension and rmtrailers) - but i haven't seen exactly whay you're looking for.
