A couple of discussions for making your own antennas can be found here and here.
Keep this in mind when building your own antenna/signal reflector:
Optimum gain for a dipole (your modem) likely occurs with a reflector (back of a can or face of a sheet of aluminum) between .15 and .25 wavelengths behind the center of the modem. At 1900 Mhz (Sprint) a wavelength is about 6 inches, so this is about 1 to 1.5 inches spacing. At 850 Mhz (Verizon) a wavelength is about 14 inches so it would be a 2 to 3.5 inch spacing. You would use this spacing for any type of reflector even if it was a pie tin.
I found that if you have a USB modem you can put it on the end of a 15' extension cable and move it around to find the best signal. Most of the newer modems have very good on-board antennas and can pull a great signal this way. Getting it away from your desktop or laptop and all the RF interference they can emit helps
I started out using a Wilson Trucker antenna mounted to the side of my house.
I am now using a directional "panel" antenna. It has better gain and I don't have overlap problems from other towers. I'm in a fixed location so I don't need what essentially is a "mobile" antenna
Unless you have extraordinary difficulty finding a suitable location to mount a directional antenna or don't know where the tower is, you don't need a omni. Get a directional.
John, over at dslreports.com has a very good comparison of the yagi, grid, and panel antenna.
Tips for building a 'cantenna'
Using an old satellite dish for an antenna
