They will sag even more. His shelves sag at 23" but the boxes help keep things reasonably workable.

If you want to go with wider shelves, you have some options.

One option is to make a double-sided shelf: Top is a piece of 1/4" thick PVCX, then attach a wood frame of 1x2's to the shelf like the original poster did for his bottom shelf, then attach another piece of PVCX. This takes twice as much PVCX but you can make the shelves 3ft wide with no worries.

The great thing about the double-sided hollow shelf is that you can heat in the shelf under the top piece so your boxes never slide directly on the heater. Plus the mass of the shelf makes for a smoother spread of heat.

I used this idea for a baby rack. I used 3/8" thick pine molding instead of 1x2s because I didn't need alot of strength. Worked out way better than single PVCX shelves. No escapes. Heat cable inside the shelves, so no exposed heater.

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