Back at the Bed bath and beyond, we had to drag around melamine shelves for fixtures and crap. They were really heavy, and 4 feet wide.

They we're tough though. We had marble bath stuff (like toothbrush holders and tissue boxes,) the boards held. We're talking close to 150 lbs per shelf, but we also had these nifty metal brackets on the front of the shelves to prevent the bowing.

Even when we didn't have them loaded down with weight, they naturally bowed under their own weight.

I'm not exaggerating how heavy they were. I worked in freight processing. I lifted, carried and opened heavy boxes all day long. These things were large and awkward for me to carry.

I'd try for some simple plywood. It's way cheaper and nice when sanded.