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That the edge is convexed, is not an issue and actually a plus, the edge should be thicker and stronger.

But convexing by flexing the substrate, varying of pressure, is problematic, because it is...

Thorlabs.com is the best source for lapping film, comes in 9x13 in sheets. Cut into 3 in sheets with a paper cutter, you will get 4 pieces from a sheet, so it is about 35 cents per piece.

Buy the...

A wet piece of copy paper under a piece of film will contact only the bottom 1/3 of a bevel from the edge. So leather, thicker and softer will flex much more convexing the bevel.

Using a high...

Yes you are convexing the bevel, just use a 3in piece of glass and water to hold the wet & dry. It will hold it to the glass fine.
You can lap the glass plate with 320 wet and dry or a diamond plate...