I have lost a few lower spring retainers during bashing and at the race track and here is what I've done to prevent looseing them again.

I just used a piece of 24g telephone wire and ran it through the hole in the retainer and around the lower portion of the spring, then twisted the ends together. Simple, cheap and works.....

http://www.flickr.com/photos/89998545@N04/8180288096/