I personally hate EXTENSION MACHINES which is what at least you and I are talking about. They are like Borg implants on a bass. Actually let me correct that, I hate the idea of any metal extension. There are other extensions that are not machines that have gates/capos/latches but they are made out of brass or other metals - I don't really like those either. My bass is made of wood - I want an extension made of wood. I personally think the most elegant design I've seen is Bill Merchant's - a lot less obtrusive than most others.
http://www.merchantbass.com/acoustic/cextension.htm
http://www.merchantbass.com/acoustic/cextension.htm