50lt boxes - (measurements on Really Useful Boxes site)
Heat mats under each box, cable if using loads. Plug all your heat mats into an extension lead, plug extension lead into your stat, set the probe on the middle mat and monitor temps in the top and bottom box. You'll have to fiddle around with the temps until you get them spot on but it's not too hard usually.
Argos sell a shelving unit which is perfect for 50lt boxes and is about £40, you've got room for about 5 boxes on there. Any more and you'll have to build something yourself. PM me if you want photos of my set up for reference.