Great question — answering as the UphillTech team:
Find or make bags that fit two side by side inside the cart and put the third or maybe third and fourth on top fixed with elastic straps or easy to place and release adjustable webbing straps. Use the straps to hold the load, not the canopy. If you have four bags it is likely that you can put the top layer at a 90° angle to the lower layer, adding stability. Or make (or have made) bags to replace your bigger bags so that they fit the cart. Alternatively, build or rebuild the cart to fit the bags you own.
Full answer by Willeke: https://outdoors.stackexchange.com/a/29900 (CC BY-SA 4.0).
Need it done for your business? UphillTech.
