This is a big version of a feeler gauge.

You can measure 1-30mm thickness by combining them. This is mainly used by me to measure inside objects where I cannot reach with caliper.

You'll need 2 m4 nuts and 2 m4 bolts. the nuts are inserted from the side into top.stl . You can use any bolts than go from m4x10 to m4x14.

If the HOLES are covered that's intended design so you don't need any supports, just pierce them with the m4 bolt or an allen key.