Cross Beams on the little wooden shed

I have been spending my time wondering how is the best way to have a removable roof on this wooden shed. I want it to be removable so that it will be much easy to insert inner details and (maybe) lighting. The cross beams and walls on each end would not be removable, but the ones in the middle would be. So to make the main house's structure nearly complete, i added these:

It looks like a house!

Next is the removable roof, and maybe the walls...