Sorry for the late reply I didn't check this for a while.

Thanks for the replies. The problem isn't that I can't find a board to put on the back, like pegboard or foam. It is that the rack is designed so that the wires are coming out the BACK. The stoppers for the tubs are screwed on the shelves on the BACK. So the back end of the rack isn't a flat surface that I can place an inflexible board on. Basically it's like a shelf, but with no back, and where the back is, there are wires and screws for stoppers etc. so its not flat. If I were to redo the rack I would drill holes in the board but the wires everything is connected before I thought this was a problem so I am trying to find a way to solve it.

The feeding problem of the two non eating snakes have been resolved. I have always checked the temperatures with my heat gun on a regular basis like 3 to 4 times a week so they are not the problem as well as check humidity with a digital hygrometer. There is just the one that is eating one week then skipping two weeks eating two weeks then skipping two more etc. I am thinking that it needs to feel more secure. I put hides for her but it doesn't seem to help, she just uses under the paper as a hide instead even with the hides. I also tried crumpled newspaper inside as "hide" everywhere no help. I am thinking if I block out some light from outside it might help. That is why I wanted to try to cover the back of the TUB as it seems so difficult to affix something on the back that has wires coming out. Maybe I'll try a piece of card as someone above kindly suggested.