The workshop's roller-shutter door comes from a Miele vacuum cleaner bag. My wife had emptied the bag into the compost bin before I cut this piece off, so luckily there was no danger of my spilling dust all over the kitchen floor!
The roof is removable, as is the door.
You can see how the walls were reinforced with L-section offcuts from the edges of the mushroom punnet. Although the plastic is pretty thin & flimsy, it forms a surprisingly rigid box when it's all glued together. I used cyanoacrylate superglue to stick all the bits together, and then sprayed the whole thing with £1 shop grey car primer.