Aluminum Honeycomb Grid Core, 1/4" Cell, 18"x18"x .500"

Mine had a decent hump in the center. If I flip it over it's better, but still not totally flat. The really annoying part is how it's cut. It's square, but the metal was crooked when they cut it. So at one end you have a solid honeycomb. As the cut continues, instead of going straight though one honeycomb row or column, it angles across five of them. So, yes, it's square, but it looks crooked and can't be used for alignment. I did not notice until I needed to trim, off one inch. I started at one end, marked an ince, then went straight across, staying in the same row. At the start I had an inch. At the end I had 1/8". If I had not cut this I would be returning it.

Served its purpose

Aluminum is to thin can't hardly touch it without bending