I made one this year and bought all the materials from B&Q
Turned out great in my opinion, and cost far less than buying a premade one that would not have fitted the space.
Dowel was about 9mm IIRC, and the height of each opening 300mm.
Worst part was loading it up for the first time and waiting a few days for it to come crashing down. It's mounted on a french cleat.
2nd worst part, gluing the dowels in before deciding to paint the frame green
EDIT: Just found the receipt. 8 2.4m lengths of 9mm dowel was £19.