Slight change of plan. I decided to use the 10mm plate to make a nice set of clamps to replace the quickly made scrappy ones. I've been using odd bits of aluminium plate that were supposed to be 'temporary' but ended up being used for ages.

There is no hard and fast rule for the dimensions but here was mine:
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Here are the 4 small clamps I made out of that 10mm plate:
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The came out pretty well.

Now I'm using those clamps to hold the workpiece for the precision vice clamps:
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Here is the drawing for them:
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Here is one roughed out as a trial:
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Looks like it will work OK:
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Just need a set of 4, then I can use the vice to machine the holes in the side of the plates.

Anyone else made their own clamps for machining, especially if they have any extra / time saving / novel features ?