All my wheel spacer knowledge comes from the BMW world, but I don't see any reason the principals shouldn't apply (just some actual numbers)
Be careful with mid-sized spacers. With ~5mm, the hub has enough lip to still keep the hubcentric nature. If you start going in the 6-12 mm range, there's not enough hub lip (or no hub lip) actually contacting the wheel, and it will not be hub centric. The biggest problem with this is the shimmy that can occur when the wheel is even SLIGHTLY off center (which can/will happen). If you go in the 12+ mm range, the spacers start to be able to include their own hubcentric lip.
Now, there may be lip extensions that fit inside the hub lip and extend it that way. A quick google search, however, did not turn anything up.
See how the spacers jump quite radically in thickness from the first two to the third? That has to do with the lip hub/centeric thing.
And here are the extenders sitting inside the spacer with no lip. You can see why they CANT put a lip on medium-sized spacers. Where the lip would have to go, there's already the bitty piece of the hub there!