I looked at this and thought it was good enough to make but I couldn't see where I'd store it when not in use so I made a de-mountable version of it. To do this I made it in sub-assemblies and then bolted it together.
You will need, as stated in the original, lots of cardboard (we've just moved house so I had the luxury of picking a big stack of identical strong boxes), Sanley knife (box-cutter), hot glue gun, rulers, pens, etc. and then as well, lots of nuts, bolts and washers. We went through nearly 60 nuts and bolts and 120 washers.
Step 1: Making the triangles
I used 1/2" bolts with 13mm heads and nuts and a large washer on each side. Using two per join, and leaving gaps for the window and door, in total there are 29 joints, using 58 nuts and bolts and 116 washers.