I don't entirely understand the layout you'd like in the room. Could you sketch up a simple floor plan?
Just to be clear, you don't necessarily need to put the rear speakers in the back wall. The speakers should be placed by degrees and depending on the dimensions of the room, 135° could be in your side walls or corners.
For example, I am planning a 9.4.4 layout in a room with no back wall either. (The side and rear speakers will be the same as 7.x.x)
My rear surrounds will be located (far back) in the side walls.
But also note that I am not suggesting you should do 5.x.x or 7.x.x. I'm not advocating for either per say. All I'm saying is that you should look at Dolby Atmos Studio guidelines/specs, as this version deals properly with speaker placement in degrees and
set your speakers up according to the recommended layout for the format you choose.
You could consider, since your walls are open, to run speaker wire for correct placement of 7 Ch surround, and just leave these on the wall (note the measurements on a floor plan) and build a 5 Ch layout now. That way, if you want to upgrade later, you can place the speakers in the correct spots with minimal damage.