Ok so, someone had kindly donated an old helium balloon filler tank. If you've never seen one, they look like a flimsy propane cylinder. I decided this was a good size, and that the handles would make acceptable feet. Looking through the scrap in my shed, I recalled having a piece of old motorcycle exhaust, with baffles still inside. This I thought would make an excellent chimney (and the baffles would stop the sparks). For the forced air intake system I used what I think is an old boiler fan (I have had it for probably 20 years, it has scratch marks on the rotor I remember making as a child). This is coupled to an old pit bike air filter which increases the pressure a bit, provides a convenient way to connect, and stops any hot sparks being pulled down to the plastic fan when it's turned off. The bit of pipe is part of an old pedestal fan, which I have previously used to practice welding.
Parts collected, I firstly drew around the chimney base and cut the hole, I found starting off with an angle grinder, just until through the metal enough to get the jigsaw in there makes for a controllable cut, especially when it needs to be a round hole. I then cut the door, and pipe entry at the back, using the same method.
Once the holes were cut and de-burred, I started welding on the peripheral parts. I feel I need to apologise for the state of my welds at this point. The are strong, but look awful.
The door was the only tricky point here. After spending 20 minutes looking for an old hinge, I gave up. I cut 2 slots and welded 2 washers into them (leave the bolt through the middle it's so much easier), then welded a second pair of washers to the door edge. Slid the bolt through both pairs and the hinge works. The huge bolt handle was a bolt already minus it's threaded part, so I just welded it on.
The net result, well it works (I did pansy or and screw it to a board), but the fire comes out of the door rather than the chimney. Though it does have a really satisfying roar and a gentle tick, tick, tick of the fan.
So far over about an hour, it's destroyed a full carrier bag of documents. It gives a total burn (no little readable bits left) and you can shove whole books in! Not bad for an afternoon and some scrap metal.