I settled on a 24 x 24″ seat module, since that seems to be a standard foam cushion size, but it also tessellates quite nicely into a full sheet of OSB. The storage volume of each module is huge at about 92 litres. The seats fit snugly but simply lift off for access. I used cheap materials for the frame but chose good quality foam. The cushions are made with 6″ of medium density Reflex foam, topped with 2″ of memory foam, so that the seat is soft but firm enough to prevent major sinkage. The arms and backs are covered in 2″ of a softer foam, which could probably do with being a bit more firm; although the additional back cushions fixed that. I added non-slip feet to the back, and slidey feet at the front. That way the modules don’t move when you sit down, but can be easily moved around when picked up from the back. The whole thing is upholstered with a blanket from Ikea.

I also built three more modules (not pictured) which enable more layout combinations, including a pretty decent bed.