I can think of 2 ways to make one, both using elastic, since you can't get to a store right away.
1. Simply make an oversized "scrunchie" -- make a tube out of fabric, at least 2x the length of the collar, cut a piece of elastic to size (it should fit loosely around the kitty's neck, w/o any stretching, plus seam allowance); pull the elastic through the tube and sew together.
2. A little more difficult -- use webbing or even ribbon (won't last as long, but he'll be growing). Make a loop of elastic (or use a small pony tail holder). Put each end of the collar through the elastic loop and sew.
For cats, collars are only needed for ID and/or bells. If you want to put a leash on the cat, you'll need a harness.