Usual method on ferries is:
Bike on centre stand. Rope from boat joist to rider’s footrest, wrap it round and back to joist.
My advice is to take a towel or old rag with you and slip it over the seat and side panel before they strap your bike down. An oily rope which is anchored to your footrest does wonders to rub the paint off your side panel and dirty your seat. If you get chance, strap your bike down yourself and let them check it.
They do have wooden wedges which they put under the tyres on rough crossings. Find a few of these and put them under your tyres just in case. Even the calmest of crossings can get choppy in the middle of the channel.