Holding the fire in place. Half a drum is traditional and (relatively) quick and easy compared to building a brick firepit. Drums are found in ditches. {img src=http://www.martinhill.me.uk/tikiwiki/show_image.php?id=188 width=50% link=http://www.martinhill.me.uk/tikiwiki/tiki-browse_gallery.php?galleryId=14&offset=0&sort_mode=name_asc}
Cut in half with a grinder. This one held petrol so we filled it with water so we didn't have a big fume-filled fuel tank to go bang when the grinder produced sparks. Make sure you have a few cutting blades (4+) and the spanner to change them...
An old lamp post and some ordinary B&Q fence post supports to hammer into the ground.
Bolted on utility hooks at several heights to vary cooking speed. The hooks did alright on the night but not really up to the strain long term, get something more heavy duty.
For heights, start at 12" from the charcoal and 6" intervals up.
Spit is some gas mains pipe (I think). To stop the lamb rotating, predrill it with a set of holes and bolt the lamb on with long studs and oversize washers. The tendons at the back feet will stay fastened, and the ribs and shoulders will also hold. A circlip can stop the pole sliding along the hooks, and a steering wheel and rope to hold it in position. {img src=http://www.martinhill.me.uk/tikiwiki/show_image.php?id=189 width=50% link=http://www.martinhill.me.uk/tikiwiki/tiki-browse_gallery.php?galleryId=14&offset=0&sort_mode=name_asc}
Like all charcoal cooking, go easy at first - if you get it to where it's sizzling quickly it'll burn quickly. It's not symmetrical on this kind of spit so you need to be able to move the whole spit up and down. On the other hand, we probably had it too slow and had to add some lower hooks. If you do have a back-wall to the fire I suspect it'll make a lot of difference, keeping the wind out and channelling the rising heat past the carcass.
I'd recommend about 1-2' from a semi-spent charcoal bed, and allow 4 hours cooking.
(If you want it done faster, just joint it and put it on the grill. It's just not so interesting)