Physics (& maths) says to locate and fix (ie make stationary) any 3 dimensional object in space you must have a minimum of 3 points fixed in space.
My preferred method locates the bottom of each wheel. These are relatively stable, if hard to attach to. They are also far apart, which maximises the leverage the points have on the bike's mass. You could use the wheel axles instead of the wheel bottoms.
The problem with the third point is that it should be as far away as possible from the other two points to maximise it's leverage, the top of the handle bars is good, but this point acts through the bike's suspension so it can and will move. By tying this point towards the front of the motorcycle some of the front suspension travel can be taken up. If this point moves it hopefully will remain under the tension of the remaining front suspension travel.
Place the motorcycle on its sidestand on the transport where you want it to be.
Tie the bottoms of both front and rear wheels to the left and right (but slightly away from the centre of the motor cycle) such that:
1) the wheel cannot move left or right (even if the transport goes 'weightless' and the motorcycle leaves the deck)
2) the motorcycle will not move backwards or forwards (may move a little under tension of the ropes)
This means 2 tie points per wheel as shown in the diagram. [Green lines to deck, purple line to stop wheel rotation.]
Then loosely tie the top of the forks (or the bottom of the handlebar if that's more accessible) on both sides downwards at an angle of about 45 degrees. Tension the rope on the side opposite the sidestand so that it pulls the motorcycle off the side stand (compressing the front forks). Retract the sidestand. Now tension both ropes so that the motorcycle is vertical and the front suspension is compressed.
This means 2 tie points as shown in the diagram. [Blue lines to deck.]
If any single rope fails, the whole fails. There is no redundancy.
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