This is the headband adjuster thing which I used during the make of LoboCNC's Blinking Iris Goggles http://www.thingiverse.com/make:263519 (I could not upload it as part of my "Made One" post so put it here as a new Thing).

It is a simple adjuster which will accommodate a 10mm wide elastic or fabric strap. Fitting - take a look at similar things which you are likely to have around the house - bags, binocular cases, camera cases. Hope it is useful.