What you need
- Clear or other 4G Desktop Modem. Series M & G work fine. $10-20 Used. (Try Amazon Ebay, or Craigslist).
- Wifi Access Point. Picostation, WRT54G, anything that says 12V should be okay!
- Wire, battery clips, diode and fuse
- 12V Battery
- Splice two power connectors or (if supported) a passive power-over-ethernet cable together
- Add reverse polarity protection - A fuse in series with your circuit & a diode in parallel (read about power protection circuits)
- Connect leads to battery clips
- Connect Clear modem to Wifi AP
Notes on Clear
Clear customer service sticks to a script, out of the ordinary requests will result in you being transferred from person to person and possibly denied. If you find yourself stuck, or they won't activate your used modem message @clear on Twitter --- reps there provide fast, competent resolution.
In addition to $50/month service, Clear offers mostly unadvertised $10/day, $25/week passes, which may be more economical if you are only using the device for events.