The dwardio ‘Go / No go’ Gauge. Held against your thicker material it also displays the shim thickness needed to bring it into focus range. Very handy! 
Edit; A scale on the inside edge for direct measuring the needed shim would be cool!
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Okay, one of the best/most practical ideas! this just jumped to the top of my “when it gets here” list! 
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This is so incredibly useful and simple!
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So many dirty jokes… so little time!
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I can’t wait to have something that will make this for my keyboard. wink
OMG how did I miss this…its brilliant!!! Your wording on this made me truly LOL
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