So you buy a nice two shelf bookcase but it’s wobbly and you know it won’t hold much. I’ve recently gotten one and my solution was to put L-brackets on it. After installing about 8 of these brackets at the cojoining parts of the shelving, it is now completely stable and ready for use.
I know this is a weird answer, but try sleeping in a good drawn end hammock (think an Eno hammock, but try to find one that is longer). If you sleep diagonally, it cradles your body amazingly. I’ve been sleeping in one for a couple of years, and it’s both gotten rid of my tossing/turning and helped pretty drastically with my back.