I think a basic need is not to have the scenic background having a right angle in the corners where the back of the scenic section meets the ends of the section. I generally cut the baseboard to a curve of about 4” radius at the corner, and fit the scenery round this. I have tried making the back to fit a half ellipse, but this is wasteful of useful corner space. The actual backscene, printed or painted, needs support, and I do this with a 3mm hardboard sheet. At the curved corner you can curve the support to fit round the corner if you give the hardboard a prolonged soaking, then hold it to the required form and let it dry out. The other thing to watch is if the scenery has a straight horizon, to break it up with trees, buildings, or hills at the corner, or it does look odd.