I don't know if I'm too late with this, but I had seen 12mm-17mm hex adapters. These were no extensions, just hexes that went over the 12mm ones. Then there are these washers that go on the outside and gets fasten with the bold.
12-17mm adapters
6 KiloWatt A123 Racer
GTP-Pletty Big Maxximum+RX8. GTP-C50-6L Hacker+RX8. CRT.5-Pro4+ZTW esc.
24s2p EVG SX 49.6mph Ebike.
18s4p Raptor 60mph Ebike. 11.5KW