If you proceed with that idea, you may want to consider that any weight that you put in it will raise your center of gravity. Also, anything that you put into it must be securely tied down. So, the idea seems sound enough if you do not traverse on any inclines.
I would like to suggest something else though that may serve you better. A cargo rack attached to a 2" receiver on your 3pt might be put to better and more frequent use. I use one on my 3pt for carrying heavy loads and sometimes to transport tools to a job site. I also use it as a platform for my sprayer. I also use the cargo carrier on the FEL frame with a quick attach 2" receiver when the bucket is not quite the appropriate thing to use for a particular task.
If the Haul Master Roof Rack is cheap enough it can be used with a sheet of 1/4" plywood as a canopy, I suppose. However, you may be able to locate a used golf cart top for a canopy, as I and others on this site have done. I paid $20 each for 2 used golf cart tops from a golf cart dealership in Tacoma, WA. I am sure that if you search around you can find one near you for a reasonable price and fabricate a frame for it and use that as a canopy. I suppose that you could also carry a light load on the golf cart top too if you wanted to still work on that idea.