I made mine a few years ago using this beta before the commercial options were available: http://www.alpinedave.com/leashless_rig.htm
I'd still build my own again today because it seems like the commercial devices are making this more complicated that required. Remember that you're building a simple backup for dropping a tool, not a NASA moon-landing. So keep it simple and don't bother with the heavy spinner unless you do some really funky swapping of tools. If you're weight conscious then get yourself a superlight mini locker
for the belay loop end and 2x mini Metolius biners
That's it... works great and is strong, light, and cheap!