A simple way to print a flexible sheet.

The object itself is just a cuboid, 200 x 200 x 2 mm. Scale as you see fit. The pictures show 200x167x2, made in black PETG. Should also work with PLA, but I've not tested this yet.

Here's the cunning bit: in the slicer, use 15% GYROID INFILL, NO TOP LAYERS, and NO BOTTOM LAYERS. I used 2 vertical perimeters (walls).

Check the orientation of the infill on the bottom so you can remove it cleanly - flex the plate and slide a metal ruler underneath.

Naturally, feel free to experiment with these parameters, and the thing itself can be any shape you want.