We have an aluminum platform cart with a removable handle.
Attached to the deck are two 8020 cross bars. On top of those are two longerons. On the longerons are posts that support the robot. By adjusting the locations of the longerons and posts, we can fit the cart to each year's robot. The height of the posts are set so the robot just clears a standard doorway. If necessary, the posts can be trimmed or swapped out to increase or decrease the robot height.
As shown in the pic, the crossbars and longerons also make a convenient place to stick stuff when working on the robot.
The cart design is evolving. Last year the height was great for standing and working on the robot, but we couldn't go through doors. This year, we dropped the post height to fit through doors, and the robot ends up at the perfect height to work on while sitting in a chair.
Next step is to add stowage for the operator console, tools, and water for competitions. We had stowage for bumpers last year, but may leave them off the cart this time.