This is from Napalmandroses' bike parts belt idea in the bikepirates thread here.

Step 1: Acquire a Used Bike Tire and Trim Off the Bead & Sidewall - I Have a Bontrager 700x25 Here.

Step 2:

I didn't have two matching cogs, but a section of a larger cog will do for a second ring. I cut it into sections with a Dremel cut-off tool as in the bottle opener directions.

Step 3:

Thread the tire end through the two "rings" and make holes to put the master link pins through. I used a c-connector single speed link to fit the thickness of the doubled-up tire.

Step 4:

Try on and trim the end to length. The belt hooks up much like a D-ring belt. Alternate pictures two and three show another partial chainring instead of a small cog, trimmed, with two teeth removed on the top piece.