Hello, Baz!
Walls are like houses, mostly trial and error. If you want to make environments seem more 3D, I would recommend the walls in “REMAP Walls 1,” “REMAP Walls 2,” “REMAP Buildings,” and the Inquisitor indoor set. If you just want to do a frame, I would recommend void blocks, the forest blocks in “Inquisitor Outside,” and literally anything else you want to do! Good luck building!