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1) look up videos of "watercolor paper stretching" on youtube, and clamp the paper down if you can (I'll tape all way around a sheet to make a nice border anyways).

2) alternately use watercolor paper blocks (which have adhesive on all sides of the sheet, which minimizes curling)

3) get heavier paper (thicker), use less water, and don't let water pool anywhere.

I use flat magnets in pairs - like neodymium ones, but disc shaped.

Like these (although I don't know the exact brand of mine):

https://www.eclipsemagnetics.com/products/magnetic-tools-and...

Oh very clever! Less of a mess than tape. Thanks for the idea :)

Get thicker paper