I made a leaf bag holder a few years back, and it's worked well. Last year Popular Mechanics® published one
with twice as many parts and a need to use clamps. Mine doesn't use clamps and needs fewer fittings so since then I've been meaning to post mine. Then I figured out how to make a funnel out of old plastic board campaign signs so decided I'd get this posted while signs are still out here in the US. The rack is sized for a 39 gallon/ 2' 9" X 3' 5" leaf bag to fit tight enough so it won't fall down and the legs will pivot to hold the bag taught.
Once the leaf bag is full, the rack pulls right out. I usually set it down with the legs up so I can slip on a new bag with the seam parallel to the long side.