I made my own with a couple of angle iron bed frames that I bought at public auction for 50 cents , some bolts, a couple of pieces of scrap wood, and paint remnants from my garage. Total cost less than 10 bucks.
I haul everything up there. My canoe, camping equipment, I have had an entire bale of house insulation up there. Even 12 bales of straw.
I mounted mine to the rain gutter. attached it in 6 places. each end and in the middle.
Mine easy handles over 400 pounds.