Before I married my current husband,I had a duplex I rented with my 2 small children. I had no extra money and little space. I was renting and could not nail anything. I bought several square concrete blocks. They were like 12x12 and 4" thick. I laid 2 down and placed a 1"x10" board on top of it to make my own shelves. I made 3 shelves in a coat closet. The top shelf held my portable sewing machine. This closet was only about 3 ft wide and not very deep. Maybe something like that would work. It is solid but temperary. Back then the blocks were about $1 each not sure about now that was over 40 yrs ago.