An A-frame structure is relatively easy to make and can be permanent, temporary, or collapsible and portable. In the latter construction, the triangular ends would be removable and the two sides hinged at the top; thus, it can be partly disassembled and folded for storage. It must, of course, have a vent or vents at the top, which is easy to accomplish at the apex where the two sides join together. If the structure is permanent, it should be topped with a ridge vent. You can make a walk-in smoker 8 feet long with five sheets of plywood, and a few 2-by-4s. For a permanent structure, of course, the outside plywood should be covered with shingles or some weatherproof material. For a really nice structure, consider using natural wood siding instead of plywood. Note that you can build a worktable and shelves at one end of the structure.