This is my home built effort, a tad crude & heavy, but practical.

It embodies some design ideas you may like to consider.

(1) Base is wider than top platform, the angle of the uprights matches that of machines' legs. This gives optimum lateral stability which is a serious problem with "normal" designs.
(2) There is a "swing up" bolster at the back attached by hinges. This is to overcome the annoyance of the machines' front leaving the floor well before the rear, meaning more pumping the jackhandle and more longitudinal instability.
(3) Onboard stowage for jackhandle.
(4) Zebra safety markings on the front (to stop clods like me banging their knees)

I completely agree with all swivelling and decent sized rubber wheels after bitter experience with smaller nylon ones(now replaced)