You'll need a few minimum basic fabrication tools;
- small angle grinder with cutting and grinding discs
- drill with a few bits; 6, 8, 12mm (the 6mm is for piloting holes)
- jigsaw
- welder (either mig or stick) capable of welding up to 4mm
- a hammer and a couple of small pin punches and a dot punch
- a couple of files; a half round and round will do
- a big hammer
- a rule, marking tools and a tape measure
A few other tools that are pretty useful (but not essential);
Slitting blades for a small (4.5 inch) grinder, they're thinner than a normal cutting blade and so quicker. You'll probably have to go to a proper industrial supplier, the local DIY superstore more n likely won't stock em.
A 9inch grinder is well useful, saves so much time and effort (sorry for the upside down photo, the uploader seems to hate this particular photo and won't show it any other way no matter the orientation of the original - sodding technology huh?)
And slitting blades to match, BE CAREFUL, these puppies chew through steel like a hot knife thru butter. They will not hesitate to take fingers off. You have been warned.
And a flap wheel for the small grinder is nice to have, just makes cleaning off paint and smoothing edges a hell of alot easier n quicker.
Oh yeah, plenty of sweat and swearing are useful, as are plasters.
First you'll have to decommission (de-gas) your gas bottle, if you need me to tell you how to do it then you probably shouldn't be doing it. l'm not gonna go all 'elf n safety' on you but it can be dangerous, you're messing about with explosive gas in a sealed container (a.k.a. bomb). Random fact for you; at -5degrees propane/butane becomes a heavy oily gas that doesn't dissipate easily and just sits around on your patio waiting for a smoker to walk around the corner, that was (personally) an interesting (read nerve wracking) day :P
I usually use a 15kg bottle, it kicks out enough heat for a decent size room. I will do an instructional on a bigger one later but "he's" a bit special and not for now. The door sizes are a balance between useability and access. You need a decent size door for filling, too small a door and you can only fit twigs in and you have to fill it every 20min. Too big a door and your ashpan door is too small as is the grate, and the fire blows out all the smoke as soon as you open the door.Once you've chopped out the carcass clean off all new edges, l know it sounds a little anally-retentive but you're gonna be throwing this thing around on the workbench for the next few days and you'll slice yourself up pretty good. l know, trust me.
Okay, now you need to chop out a fake floor for the beast, anything above 2mm will do, as it doesn't really take any direct heat. It's just to support the ashpan and fire grate. The diameter needs to be slightly (and only slightly) smaller than the internal diameter of the carcass. When you drop it in it should just sit slightly into the bottom curve on the bottle. When your happy with it then stitch weld it in. You should just about be able to see from the photo that the base plate/ fake floor sits about 10mm below the bottom edge of the bottom doorway, this small ledge minimises the amount of ash that is dragged out when you pull out the ashpan.