My husband and I made this - if you want specifics I will try to help but basically it involved;
Ringing up a conveyor belt company and getting them to ship us 4 industrial rollers for £5 each.
Attaching the rollers to a piece of scrap wood (10p from B&Q) using angle brackets at the right distance apart (£20 for the lot)
Adding a Sofa caster on a bracket on each side to prevent accidental sliding off (£3)
Putting the front wheel up on bricks to prevent the feeling of going downhill (Free)
Adding a car jack under the frame to make it easier or more difficult to push (Also free- from freecycle)
It works like magic - the only drawback is that it is fairly noisy.
This is mark I, will let you know how the next version goes.