Supercheap recently had a storewide sale, and I bought a few F-clamps, especially the 1000 mm ones. I like the price, I like the length, but I don't like the clamping mechanism. When you turn to tighten, you tend to throw the workpiece out of the centre, which makes the clamping uneven.
I made a small add-on which the clamp works a bit like a parallel clamp.
Easy to do, and is reasonably successful.
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The first picture shows the construction of the arm, and the second picture shows it in action.