I'd secure the front wheel so the bike can't move forwards or backwards and put a ratchet-strap over the seat.
The strap-over-the-seat method is the way bikes are tied down on the cross-channel ferries (usually with just the side-stand down) and they survive Force 9 gales with no problems (sorry, I mentioned the ferries

).
Additional straps round the grab-rails would be the third tie-down spots for me.
Triangulation is the key.*
And yes, I have trailered a few bikes in my time.
* I've just noticed that rhymes - I'm a poet!
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Mike - Riding on the right - riding for pleasure!
'09 Silverwing 600 - 'The Winged Express' - Delta Blue, ABS, Heated Grips, Givi Airflow Screen, Utopia rider's backrest, Givi pillion backrest & E52 Topbox, Cortech Super Mini Tank Bag as a tunnel-bag, Starcom Digital Comms System.
www.x9ownersclub.co.uk - for all Maxi-Riders who want to RIDE!
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