I love pallets and have been experimenting with them for years. As some will know, I rely heavily on home compost and substantial quantities of green manure. My current method is to set the bins on a course of bricks, this has 2 advantages, it helps to preserve the bottom of the pallet that is in contact with the ground and deters rats from burrowing in. I like to give a one off treatment of preservative. Its most practical to have the slats on the inside and depending on the type of pallet I make them as solid as possible, filling in the gaps by nailing pieces of wood in between the slats. This improves temperature and speed of composting and if I turn the stuff once I can have it ready in about 3 to 6 months, depending on the time of year. Old carpet completely covering the compost is a must in my opinion, usually held down by wooden boards. I would also recommend a piece of old heavy duty carpet in the bottom of the bin, this stops the compost mingling with the soil, deters vermin and makes it easier to get the stuff out because you have a base to work off. The best pallets are the small ones that are sometimes used to transport brick setts. These are close boarded so need little work and they provide a nice compact bin. My bins are 2 pallets square (ie 8 pallets), but a 4 pallet sq would be ok. Like “stringfellow” I have started using tye wraps and the bins hold together fine. I find the outer side of the bin is useful for storing all sorts of bits and pieces.
Finally don`t forget that a good mix of materials is important. I use plenty of comfrey as an accellerator.