I notice that your baby is 9 months old. If not crawling or standing yet, soon will be.....so that changes a couple of suggestions.
For tiny newborns, a laundry basket works extremely well. A pad made from a folded blanket works as a mattress.
For older babies, there are wonderful threads showing modifications to the lower bunk, as mentionned earlier.
For those without a bunk, do some research into adding a bunk unit across the main bed area, either temporary or permanent, which can be either hammock style or a solid wooden platform. Some fiberglass trailers came with this option when made.
Failing any of the above options appealing or viable to you, another option is utilizing the open floor area underneath the bed, with a pad of folded blanket or even a lawn chair cushion pad.
This option is good, too, because a) the baby cannot fall
off the bed and b) no baby on the walking floor space to restrict your own movement.
Hope some of these help! They all worked well for us when our boys were little.