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Compared to a similar size DMT the Atoma is light as a feather . The plate is aluminium not steel + you will have the added bonus of keeping your stones flat.

I still think it's a bad idea. The abrasive strip is only held to the plate by 3 strips of tape.
I think what's left over will be somewhat compromised.

You can cut the sheets as described in this link:
Tsuboman Atoma 'paper'. : Tools from Japan, Japanese woodworking tools direct from Japan.

I wouldn't go cutting down a plate.