Reverse rolltop topper fixer
I've just been dealing with this issue on my trailer. I sandwiched a piece of canvas between lots of 2"x40" strips of wood to a make a sort of tambour piece to lay across the top of the 'stream... Plywood would work, I just used some scrap. To describe this better I laid out half the wood with 1" spaces between the strips, put the canvas over this wood, then put another layer of wood strips over the canvas aligned with the ones below and screwed down through the canvas into the wood below. This was pretty quick to make and conforms to the top of the trailer to distribute my weight to the ribs. If you need a picture I can send it.
This might be more complicated than you are looking for, but if you are doing real work up there and want to avoid the expense of one of those fancy ladders... I suppose the weight of the worker might be an issue too, I am about 170 lbs and my trailer doesn't seem to mind.