Corrugated iron compost bins
If you have the time and materials to build a set of three compost bins then the simplest and cheapest way is to build it out of corrugated iron.
Not only are they cheaper to build than bins made only of wood but the corrugated iron sides last longer than wooden sided bins as the iron sheets are more durable.  They also take less time to build as whole sections are covered with a single sheet.  The only disadvantage of corrugated iron bins is that they still take a fair effort to build and therefore are not as easy to move around as Plastic or Chicken Wire compost bins.   Guidelines for building a set of three corrugated iron bins are as follows:

A. size of the bins
The first thing to do is to decide how big you want your bins to be.  This will largely depend on how much compostable material your garden generates. For more information on compost bin size see Size Of compost Bins.  For this example we will set the size of the bins at one metre by one metre (40 x 40 inches). This size is suitable for a medium to large garden.

B. digging the postholes
illustration of how to make a corrugated iorn compost bin
Illustration showing  some of the key points in constructing a corrugated iron compost bin.
Dig eight post holes in two rows.  The holes should be forty five centimetres (18 inches) deep and one metre (40 inches) apart, measuring from the centre of each hole.

C.  Putting in the posts
Place eight 1.5 metre (5ft) posts upright in the holes.  Either fill with a fairly dry concrete mix or with earth and tamp down with a crowbar to make the posts firm.  As you are filling the holes use a spirit level and string line to keep the posts vertical and in a straight line.  Make sure that the distance between each post is the same, so that the wooden slats used at the front can be interchangeable with any of the bins.

The best type of posts to use are rot resistant hardwoods. In Australia the ideal posts are
Photograph of three chambered corrugated iron compost bin with removable slats at the front.
Three stage corrugated iron compost bin with removable wooden slats at the front.

Photograph of a corner section of one bin and a close up of a guide for the removable slats in a compost bin.
Corner section of one bin and a close up of a guide for the removable slats.
Jarrah or Red Gum. Treated pine can also be used, but there is a risk of some of the arsenic used in the treatment process leaching into the compost and soil.  The best post width is a 5 x 7 cm (2 x 3 inches).  Round posts can also be used, though they are harder to attach guides to at the front of the bins for the slats to fit in.   It is possible to use timbers of a smaller diameter and height, but if you do you run the risk of the sides of the bins being pushed out by the sheer weight of the compost.  Hardwoods that are less rot resistant can also be used but they are likely to rot out much more quickly.

d. attaching the sides
Cut lengths of corrugated iron to fit the rear and sides of the bins and attach with nails or screws. Note that it is better to leave the posts to settle in the ground for a day or two before attaching the corrugated iron.

E. cutting the slats
Measure the distance between the front posts. (the distances should be the same) and cut nine 50mm by 200mm pieces of wood ten millimetres (1/2 Inch) smaller than the distance between the posts. 

f. guides for the slats
To hold the slats in place at the front of the compost bin  screw two thin strips of wood or angle iron to the inside sides of the front posts.  They should be slightly wider (suggest  5 mm (1/4 ") wider) than the width of the slats, so they can slide in and out easily.  The guides only have to be the height of the three slats stacked on top of each other  and not the full height of the posts. 

WARNING Galvanised iron will leach zinc into the soil as the iron slowly oxidises when in contact with compost or soil. If you are at all concerned about this happening then do not use galvanised iron.  An alternative would be to use hardwood plinth board 15 cm wide by 2.5 cm thick.