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Take a razor you don't like.
Lay it flat on the table with a pencil underneath the heel and the toe.
press in the middle until the elastic distortion turns plastic.
release pressure.
voila:...

As much as I would like an 8x3, my 7x1.5 does the job just fine.
I only use it for polishing after the 8K, the fact that it is smaller does not hurt.
You'll want to round the corners though. I...