The formers help support the wing center section, and the cabin sheeting gives torsional strength to the fuse. The BUSA 1/4 scale shows the door on their plans and the 1/5 Sig Cub also builds with the functional door and window. If you want to add some reinforcement to the formers in the way of carbon fiber strips, it wouldn't hurt a thing, though. You can also build the wing struts to be functional, and they will transfer much of the wing loads to the bottom of the cabin structure.