I have a very similar stainless commercial fridge I refurbished to a smoker 6 or 7 years ago. I found a metal fabrication shop that had a left over chunk of expanded aluminum from a previous project that they sold to me for $60. My shelf dimensions are approx. 23 1/2" X 23 1/2" and a experienced welding friend of mine was able to cut enough to make me 6 shelves and use some scrap aluminum rails to weld to the bottom of the expanded aluminum to reinforce them to be sturdy shelves. They have worked out great and wish I could add a few more.
Mossy never thought about aluminum, have to see what we have in the scrap yard. What are you using for heat and controler?? My box still have a 2000 watt heating element that works in it, was thinking about building a smoke box on the side so I could do some cold smoking.