Everytime I'm in one of the large box stores and pass the shelving areas I find that someone has come up with another new creative shelf. Check the lumber area for planks 2"x10+" by any length you want. Support the planks with patio type blocks 12"square? Or whatever.

I just looked at a spare 3 step riser I had left over and decided that IF I remembered and bought another one and used some planks I could put my winter sowing containers in a better place than I did last year.

Another idea might be bakers racks?