There are compost bins on the plot we have taken over. They are made form a few old pallets just wired together and are a bit rough and ready. I have to confess, I made them a few years back to help out. There is good, usable compost in the bottom of each section but the heaps never got hot enough to kill weed seeds.
As soon as the beds are sorted the whole lot is coming out to be replaced by a 3 bin New Zealand bin. Again, it will be made pallet wood with a removable front, a waterproof lid and sides without gaps or holes.
There is often confusion about having slatted sides which allow air into the bins which is thought to be necessary for composting to happen. What it does is keep the bin cool which slows down the process or or stops it working. More importantly it prevents the heap reaching a higher enough temperature to kill weed seeds.
Often the advice is to use treated timber as it last longer. There are several reason why you should NOT do that. Compost bins rely on bacteria to break down organic matter and they do not want to be in contact with anything that might kill them. Also, chemical wood treatments can leach into the compost contaminating your veg plot.
The last big compost bins I built were made from recycled scaffold planks which lasted at least 10 years. There were four, one cubic metre bays and it was possible to make 12 cubic metres of compost a year! The new garden is much smaller and we will probably be making 1 cubic metre.
A good set of plans are available here but as mentioned above I would not make the sides or back slatted. I will also have a closer fitting lid.
Photos of the progress will be posted so please come back soon.