I worked on about 5 or 6 variations last night before I hit on this one which isn't significantly more difficult to cut out with scissors. In my own tests, this new shape overcomes this exploit while still using the original design that calls for two strips of 2" x .625" tamper-evident tape. (When you fold this new design up, the tape now sticks to all three "panels" in the folded area so the innermost panel can't be snuck out.)

Thanks again yellowcoin for the excellent experiment.

http://i.imgur.com/Cx4Tg8V.jpg

PS: Yes, those are live keys, but there's nothing stored in them this time. Yet.