Don't worry, your back will get used to it after a few days. When we stay in a monastery, our purpose is to practice, so if you sleep on a matress, well, it can get too comfy.
If you follow a teacher who have done tudong (such as staying in jungle or caves), the place you sleep may not certain (that's the real good part, actually). When we visited a humble monastery in Tham Lod, Mae Hong Son, the abbot had a small wooden structure to offer us to stay for the night. Else, there is always the hall:
The view here is good, though it can be cold at night.
For those who have not stayed in forests or caves, you may think that it is not possible for you to do so- eg because you have fear of darkness, or are afraid of mosquitos bite. But if you are sincere in your practice, you would find that staying in a natural setting is very good for practice. You would sleep less and tend to be more dilligent in practice rather than in a more comfortable place. The hardship may put your patience, courage and tolerance to the limit, but tell yourself that well, you're not about to travel all the way just to give up, right?
Our kilesas (defilements such as laziness, greed, anger that we have accumulated since time immorial) play a lot of tricks on us. Be mindful and see through their tricks.
But do not go off on your own into the forests/ caves/ cemetries just like that. Ensure that you practice under an experienced teacher and seek advice first.