Hey Cort,
You should use an 3mm spacer actually 
(or like 3 x 1mm speedring - but the stl folder is still in improvement ).
About that, as Timo says
Its a part of the adjustment, as I 1st would like to make it simple, in just one piece. He tested the 1st version with a Taiwan esk8 squad rider, it fits perfectly, just this little gap to complete. But reading you make me doubt about the one piece system, cause the axle is not stepped like the Kahua (so the wheel is directly pressed against the shoulder of the diameter change), and there will be a lot of force exercised on the plastic/alloy piece. There are 3 areas that the driver support against the wheel rim at different heights: the inner bearing area, the screw spacer and the external/outer flank of the rim.
Maybe it’s more doable with a CNC alloy type driver … What do you think guys?
