Here's what I did for the graded ones.
The base is solid oak, except for the bottom which is 1/4 Masonite with an oak support under the center.
The top is Oak plywood with solid trim.
The unfinished insert is 1/4 birch or ?Aspen? whatever they carry at Lowes. They have more choices now.
I will for all three companies, with a bit of space to the sides and top of the slabs.
Capacity would of course vary since the slabs are different thicknesses, But for thicker slabs (I just measured a GAI I had on the desk at 1/4 inch) It will hold about 250, a bit more for SGC or PSA, maybe a bit less for Beckett.
The stuff I'd change would be using a different lid support, The one I got is a friction one, and it's loose now. Adjusting means removing the insert, so I haven't done it.
I'd also line the bottom with felt simply to keep the slabs from sliding when the row is mostly empty. That's not really a problem, except if the table gets bumped.
I'm planning on doing something like the photos posted earlier, but sized for toploaders and modular. That will be to replace the shelf and boxes in my hobby room. And to add a load more capacity.
There's a guy who makes wooden boxes pretty reasonably and usually has some in one of the BST sections.
Steve B