How about something like this? Take 4x8 sheets of T111 or Masonite siding and put a frame plus a stud on the back, creating a top and bottom plate and one stud (24"OC). Drill holes in the "studs" to accept 1/2" x 4" bolts to connect the panels. You'll need a solution for the corners, maybe just metal connectors or a corner post and lags. After the panels are up, put on a second top plate with deck screws, staggering the joints to tie the panels together. The door could be the same type of panel, on hinges. Dimensions of the finished walls would be in multiples of 4'. Bang together some roof trusses with metal connectors, go 24" OC. Deck roof with plywood and deck screws. Stretch blue tarp over plywood and replace as it deteriorates. Easy to build, easy to knock down, all flat sections to transport. You could do the floor the same way, or just set the walls on a gravel pad and do without a floor. Use PT for the bottom plate.
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