I bought some fancy Tormach TTS ER20 collets, only to find I didn't own a wrench big enough to tighten the 30mm collet nuts. Rather than run out and buy a wrench, I got out the 3D printer and made my own.
I have a small printer (Printrbot simple) so its a stubby wrench. The holes in the handle are simply to reduce the amount of PLA used. The wrench fits down well past the hex area to help alignment and reduce the possibility of the wrench slipping off.
So far it seems plenty strong enough for tightening ER20 collets.