Hi John, this is the sort of thing I was originally thinking about:
There are only really two critical dimensions and they are the internal width of the channel (so it will actually fit over the top edge of the trailer) and the diameter of the 'peg' (or socket, so that it is a loose interference fit in / over whatever pole I select).
One of those brackets on either end of a pole sitting across the top of the trailer lengthwise to stop the PVC cover from pooling water. The trailer sits with a reasonable slope to the front so the cover does shed water reasonably well, as long as it doesn't get a chance to sag inwards too much. I may go to two or even three poles to give a better effect.
So, the real work will be being done by the combination of the inside edge of the bracket, the top and the support tube / peg.
As the trailer is only around 1.4m long, the tube doesn't have to be particularly strong but it does need to be reasonably stiff, easy / cheap to buy and ideally non-rusting. Once I find the right material I can decide if the fitment to the 'bracket' will be internal or external. I briefly considered plastic conduit but that would probably sag in the heat.
Maybe a cheap paint roller extension pole or those mock bamboo, green plastic coated steel poles (the latter being very cheap and reasonably stiff)?
Cheers, T i m
p.s. Alternatively, depending on the diameter of the tube, a simple (3d printed) 'fork' on each end of said tube (with one leg above and one below the trailer top edge) with the tube slid in at an angle might achieve the same end?