I think I have had about 4 different SI's since I started geocaching. My first one was a limited production of a metal water plasma cut disk with Haffy engraved into it.
My second one was a specially made Maine bill. Can't find a picture of that one though.
My third and similar one to the one I have now was another Maine outline with my avatar in the center of it.
The one I have now is basically the same but now since I live in SC I used the state outline as well.
I make them simply by printing them off on magnetic sheets which I can get from Wally World for approximately a buck a sheet. I get 16 of them per sheet and then cut them out. I then just slip each individual cut out into one of those little plastic baggies which also come from Wally World which I buy 100 of them for about 2 bucks. All in all I have less then 10 dollars invested in some unique refrigerator magnets for my own signature item. Which reminds me I have to make some more.