thanks for the tips, especially about only needing a few inches clearance above the smoker, and having vents top and bottom for proper drafts. A hat-tip to Jimmy-James' smoker box for his simple and easy to make vents.
It is actaully easier for me to build a box out of plywood than to collect a mini fridge and adapt it because I have a few sheets of very good quality birch plywood on hand and a well-equiped woodworking shop.
I found an extra pizza stone in the kitchen which will isolate the pellet smoker from the bottom ply, so that solves that issue.
Dave Omak - I am counting on the smoke rising and spreading through a diffuser to get the food covered and a proper draft. An air inlet at the level of the pellet smoker and and an outlet offset to the top of one sidewall should feed the smoker and ensure that the smoke has to rise to the top and then down a bit and out, which should be enough for a box a less than 16 cubic feet.
I suspect a computer fan might clear the smoke a bit fast and dry out the food, but that depends on the volume of the smoker. The stream of air from a fan might also leave a few dead-air spots. As to the moisture, the bacon and sausages I want to do will have dried out a bit in a wine cabinet, into the smoker only for flavour and then into an oven if cooking is required.
I am planning on using this smoker undercover in a verandah so no need to use any sealer or a paint. I also won't use any glues in the build - only screws, dados and rabbits.