There is
this post in an old thread which gives some good advice about trolleys generally, but your specific about a limit of 20cm is quite restrictive for the construction. I would guess that would be best resolved by using rails and solid steel wheels, since rubber-shod wheels with swivels capable of supporting the max weight of the trolley and machine are likely to be taller than 20cm to start with.
I had rubber matting on the skid lines for all my trolleys (three of them) and at least 2-3ft extra width on both sides to give a solid reference when landing. Nothing worse than hovering without a proper reference to where you're landing.
Along with the rubber matting were a series of metals straps across the rubber to ensure an earth from the helicopter to the trolley, and an earthing chain from the trolley to the ground. Nothing wrong with putting some tape onto the rubber to give alignment marks to get your helicopter in the right spot on each landing, too
Fore and aft for the skids, and one lateral to line up (maybe) the toe of the skid or the crosstube. Then you know where the helicopter will be when the trolley goes into the hangar and can position hangar support gear accordingly, knowing it will be clear of the blades, etc.
Invest in a good tug to push in and out: third hand ride-on mowers may be cheap but will they perform when it comes to stopping in a hurry?