[OXO cube]

I am using a Scaleauto gauge to measure track clearance, it is not perfect if I check it with my digital caliper, as I have found that the increments on it can be plus or minus 0.02 mm out, depending on which step you are using.

I'll have to look for one of those for future use, my metal sheet offcuts were around 0.1~0.15 mm shy IIRC

Pendle Slot Racing have them for sale

cheap ebay feeler guages also work , are accurate and only around £3