It's recommended in Paxton Quigley's book,
Not an Easy Target. See it here (under DoorJammer):
http://www.paxtonquigley.com/doorjammer.htmlI like the design of this one better than those adjustable door braces (
http://www.carvaluesplus.com/security/jammer.htm), but can't seem to find it anywhere. Any leads would be appreciated!