I made a belt from relatively heavy canvas purchased at the fabric store. You can leave a 1/4" seam allowance, sew three sides, turn it inside out and iron it, then complete sewing the final (short) side. Worked like a charm. I also left the belt intact underneath the plastic ammo belt section to lend extra strength and keep the plastic from splitting over time. The plastic section was attached using rivets.
The only issue I have with the canvas is that one of my leather ammo pouches really pulls on the fabric and creates a sag. If I were making it now, I'd add a layer or two of interfacing to the inside of the canvas.