I agree with Alyosha and Jennifer, I would normally use space. I would use slot if there was a specific way to locate something on the shelf, to slot it in. That item would have a slot. It can also be used as a synonym for cram, or find a place where there isn't much room available.
- Is there space for another book on the shelf?
- How much shelf space do we have left?
- Can you just slot it in on the end?
- Put it back in its slot.
- Do we need to create a slot for it?
- Do we need to make space for it?
- Do we need to make some room for it?
- Can you put it back in its spot, please?