How much of a problem is volcanic ash when the engine isn't running? Perhaps clean the worst away and dry motor to blow the rest through before you start it.
Whahaha. Blow the rest away? Good luck with that. Stick your finger in your chimney soot and then try blowing it away.
As for the covers, these guys do custom work for any aircraft and understand how to make stuff so that it goes on easy and stays on.
http://www.arcticovers.ca/Home.page