I am thinking of transporting my bike on a rack or open trailer

Sammy Miller used to those trials bike covers in the 1970's

Can you still get anything similar, to keep the road salt off

A waste of time as far as salt goes - too much left exposed. I had a sail maker convert a bit of tarpaulin I had to cover the whole bike, with a separate one for the forks and front wheel. Full coverage worked well.

i have been looking for one also.

not found one yet

I have a road bike cover, but is massive on a skinny trials bike

Same here, if you get a small one its not wide enough for the bars

Here you can get them:

 

http://www.ernst-kuerschner-sattler.de/Angebote-und-Service

 

 

If there from where I think, couldn't see from the link

The cover doesn't cover the bars, just the body of the bike and aren't cheap at 150 euro's