The box frame is made from 68 x 18mm dressed lumber which is dimensionally stable and basically straight. I used a drop saw to cut the sections and assembled the box sections in my garage with screws and white glue. I supported the box frame as best I could whilst screwing to the walls. At this stage the frame is not 100% stable and strong enough to support the railroad. However once sections are joined they are reasonably stable. In some areas I installed braces from the outer frame end to the wall at the skirting board and this ensured the bench was very stable.
Using a continuous backdrop is important as it can bend around corners and provide a continuous horizon with no joins. You need to seal any nail and screw holes before painting and with flexible sealers like no more gaps you can create that continuous backdrop. You can also see here the valence and the sealed joints and nail holes prior to painting.