I decided not to go down the coal bunker route, and I've built a new bin out of pallet wood, 80x80. I made three stackable 'rings', with no top or bottom, each 40cm high. The idea is that you stack them up to make a bin, and to turn the heap you take off the top one and it becomes the bottom of the new heap. Fork the compost over, move each ring in turn, and voila a fully turned heap. I can also build more rings if I need to, although 4ft seems high enough for now! It keeps the dogs out at least.
Given I live in a fairly rainy part of the world, should I be trying to keep the rain out of the compost?