Although I unfortunately don't have pics right now our cart is all custom. Our welders bent some 1 inch tubing to make a base that's wide and long enough to hold three bins plus our driver station and a battery on top of the bins, then an elevated U-shape bent from the same tubing. The U-shape is the width of the inside section of the C-Base long configuration frame, so the robot rests there. The wheels can free spin there, and we used it for testing motors often.
We have two simple lcoking casters and two fixed wheels, larger than 6 inches to get over the cord bumps at MN North Star's venue. The casters were from
Minnesota Casters, a distributor in MN. Gotta love our sponsors.