The furniture dolly works for me (all rolling on concrete) so haven't made one.
The dolly (solid) wheels are under the frame, probably could be changed to larger casters (maybe expensive) - but would still be solid and probably still a PITA on gravel..:(
But - *HERE YA GO* !!
Harbor Freight has two items that should handle your gravel with ease.
1. "Steel Mesh Deck Wagon" - 1,000 Lb capacity.
Comes with a pull handle, and has large wheels.
Can't tell from the pic if they're pneumatic or not.
2. My favorite candidate.....:C
"Rolling Work Seat With Tool Tray".
It *does* have pneumatic tires (way larger than the wheels in #1 above) - 300 LB capacity.
The "deck" below the seat (skip the seat) would be perfect to attach an appropriate size chunk of plywood - maybe framed with 2x4.
Add a pull handle of your own design.
Both are in the July issue of the HF "sale" items - page 20...:W
(Bring along a 20% off coupon - see which one will yield the best price)..;)
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