I also recalled I had an old Historex kit of a Gribeauval caisson. This had three larger wheels that were more suitable. Since I wanted four similar size wheels, I would need to scratch build one more and add the reinforcing spokes to two of the wheels. Here is the basic Historex wheel with a Perry 24lbr and a redoubt VMI cadet for size:
Here are the two wheels modified to add an extra spoke in every other pair of original spokes. These will be the wheels for the front half of the sling cart:
And showing the wheels and the HBG the sling cart will be carrying:
And an unfair question: These are the back wheels and one was scratch built. Which One?
Hard to tell with a low-res internet picture, but the left was scratchbuilt. I think it will pass with a bit of paint!
Also progressing on the big Rodman. I will use this gun for the sling cart and am planning a slightly smaller one for a sea-coast position.