this was flat before it finished drying

If you protect it between two sheets of newsprint or similar paper and run a warm iron over it, will it flatten out again?

I would imagine that once cut into bricks and glued down, you would not have a problem with it. You can use egg cartons for bricks, too.

it seems relatively flat once it's cut into bricks. It's just a pain in the patootie to draw the cut lines on it and keep them relatively the same. Have ended up with some skinny bricks and some short, fat ones. (blaming it on the shape of the paper, not my cutting/drawing skills.)

I know about using egg cartons. I'd prefer to go that way but the store that I go to doesn't carry anything but the normal kind. I don't go out to eat enough to get enough from fast food drink holders.... so improvising and trying to make my own. It's an exercise in patience.