I also recently added some bulky items. I put in a shelf similar to yours, except it is almost full width: 26 inches inside. It consists of two 30-inch plywood boxes, open on both ends. Placed together, they create a 5-foot-long tube. That leaves 3 feet at one end for some tall stuff. I used 15/31" plywood and L-braces to tie the pieces together and give it strength (although heavier stuff goes on the bottom).
This (almost) doubles my usable storage space, and stuff that was previously on the bottom of a pile are now easily accessible. I created two boxes because one long box would be heavy and awkward to handle.