Wheel spacers of one of the biggest rip-offs going.. you could machine one with a CNC machine for £3, but they sell them for £70 or something daft.
I was speaking with a friend here at work, who just happens to co-own a CNC plant in India... he said if I can get a spacer he can mass-manufacturer them and have them shipped over to make us both a few quid.
Personally, I would go thicker than about 12mm with longer bolts, after that you need the 'double hole' type which bolt to the hub, then the wheel bolts to the spacer (if that makes sense!?)
Rich