If you want a flat ramp with no supports it will have to be of steel, probably at least 3/8" and that sucker will be extremely heavy. Also, you will need strong anchors where it latches to the shed because that is where all the force will be applied.
Normally, for something like this you always have support of some kind.
For a removable solution the obvious plan would be to use ramps. Landscaping equipment suppliers sell ramps for getting mowers and stuff into trailers and some of the ramp systems that have two tracks are relatively light.
Do an Amazon search for "trailer ramp".