Raspberry Pi 2", a tiny computer that fits in the palm of your hand which costs roughly $35. You need two network connections, one to the Internet, and one to your victims. I chose Ethernet to the Internet, and WiFi to the victims.
The setup is shown above. You see the little Raspberry Pi 2 computer, with a power connection at the upper left, an Ethernet at the lower-left, and the WiFi to the right. I chose an "Alfa AWUS050NH" WiFi adapter, but a lot of different ones will work. Others tell me this $15 TP-Link adapter works well.. You can probably find a good one at Newegg or Amazon for $10. Choose those with external antennas, though, for better signal strength. You can't really see it in this picture, but at the top of the circuit board is a micro-SD card acting as the disk drive. You'll need to buy at least a 4-gigabyte card, which costs $4, though consider getting an 8-gig or even 16-gig card since they don't cost much more.