How exactly do you make one?
Easy?
Radius?

Any way to make a device that jams Wireless Internet?

Its way easier to jam wifi than cell phone. All you need is a wireless ap that can run ddwrt, and theres a way to get it to hop through the channels while constantly broadcasting a deauth attack. Maybe its a plugin or maybe its as easy as a few commands at the admin screen, idk look around. Rig that up in a backpack with some batteries.

Schematics for everyone!
http://blog.tayx.co.uk/2008/03/10/cell-phone-jammer-schematic/ :
http://www.tayx.co.uk/blog/wp-content/uploads/2008/03/cell-phone-jammer-schematic.gif