How about a couple of these:
http://cgi.ebay.co.uk/RUBBER-TOW-BAL...1%7C240%3A1318
Cut the square part of the bottom and slip the rounded portion over the bush / washer / nut
OR
http://cgi.ebay.co.uk/BMW-ball-joint...1%7C240%3A1318
A ball joint dust cover - leave the nut cover on, and then stretch it over the bush / washer / nut. You may have to enlarge the hole in the middle though.