Does anyone have any suggestions for a bike cover to use when transporting a trials bike on a car bike rack? 

I'm wanting something to keep the salt and road grime off the bike when transporting it in bad weather.

There's loads of options for road bike storage but I can't seem to find one the right size for a trials bike.

I would have thought a scooter cover may work, unless it's too short?

I bought one from eBay “Ryde small motorcycle cover.” Only £8, covered trials bike well and survived motorway journey from Kent to Northumberland in the rain. Not completely waterproof but kept all the road muck off

I don't see the need for a cover whilst it's on a rack. Surely you'll wash it down when you get home, and if it is cinched too tight to stop it becoming a parachute, it chafe on the alloy or plastics and stickers. Every bike I own goes on a trailer, no cover, and washed when at home. And that goes for my road bikes too, when transporting them long distance. It'll be hard to disguise a trials bike on a rack with a cover, but, each to their own. Whatever works for you. I just let mine breathe.