I thought it might be helpful to post my $2.00 thrift store holder for Spectrum Noir markers (plus the few Copics I own). Yes, the holder is curved--I did not use a fish-eye lens on the camera.
I used double layers of corrugated cardboard (cut up cardboard box) glued together just to give each "shelf" added strength. I glued (with Score Tape) a couple of pieces of white cardstock on the back of the holder to keep all the markers from sliding too far back and to make them all even in the front.
In one of the photos I took the top two rows of markers out just to better show what I meant about the cardboard shelves.
It may not be pretty, but it works great!