rubber band, rubber o-ring, mark from a sharpie, tape.

if you go on ebay, you can find various clamps in almost any size and color, advertised for use on bikes, just search for one that's close (if you can't get it perfect get one that's a little small so you can bend it open; if it's too big you can't make it smaller if it's metal), and just use it for whatever purpose you see fit.

this would do the trick, it stays in place on the seatpost and doubles as the actual clamp as well: KCNC SC 13 Twin Seatpost Clamp Cycling 30 7 27 2 31 8 27 2 34 9 30 9 34 9 31 6mm | eBay