Hi Dan
In his post Pete_b said buy a topper off Ebay, there is no need to do that all you need is an old rear axle from a car or van.
Turn the diff downwards to face the ground and fix a blade about 3 feet long made from a good thick piece of steel, then where the springs would normally fit on to the axle either weld or bolt a couple of lengths of angle iron, bring them together and fit a tow hitch and bingo one cheap as chips topper that you can tow behind a car or 4x4, a guy I used to fly gliders with years ago used one of these for about 15 years and it never let him down, you could leave it in the field and chain it to a tree or something to stop it being pinched.
You could go mad and fit some safety guards if you wanted to, LOL
Hope that helps you
Dave