Hahaha ive been thinking about it a lot since I see the program.
I would line it with aluminium sheets not steel if that's what you was thinking.
I only thought of smoking as an after thought as I love a bit of smoke on my meat now but yeah they are just a big oven, I was on the understanding that the box as a whole with bottom was to retain the heat to make it like a conventional oven, enclosed unit.
Either way I think the idea is awesome and whatever the size its a bit of garden theatre like the old beer can chicken. Although pulling a whole cooked pig out of a box is a bit better than a chicken standing up in a smoker with a can of beer up its bum.
I'd definitely dry rub the meat side and inject through the skin, bit scared of the price of pigs maybe a smaller pig is a good idea to start with. I'd only want it butterflied no other butchery.
On one video they said the head and brains was the best bit, now ive tried a lot of things, frogs legs, snails, ostrich, zebra and crocodile but pigs brains really do not do it for me!!!!!!! They also said to take out the kidneys if they are included in the pig.
Ive got an old butler sink I dug up in my garden, like you I think i'll try a cheap version first, making a box out of ply, no handles or legs just sit it on the sink. Foil the inside and maybe just buy some dirt cheap cooking trays for the coals to sit on and some for the pig to sit on, maybe just get a big chunk of pork even half a pig and try it out.
Wish I hadn't of seen it until the summer as i'll spend the next few months winding myself up wanting to do it