I havent used the crossing you are using, but I cross the channel regularly between the South coast and Brittany. My Harley is stable on the sidestand (hobsons choice really coz that's all its got) so I put it in first gear push it against compression then put the stand down. Usually a strap or rope over the seat (provided by Brittany ferries) and tied to the hooks in the deck is sufficient. They usually supply padding to stop the seat being damaged or if not I use my gloves or overtrousers. On my BMW where the sidestand is crap I have to use the centrestand, but a rope over the seat does the job again, although I never feel it's as secure as the Harley. Sometimes you have to tie them against the side but most of the time they wouldn't move anyway. Often the deck crew will do the securing but you might feel more comfortable checking how they've done it.
If you think you are too small to make a difference you have never spent the night with a mosquito.