This design of compost bin came from Mark Wells, a friend of mine. The design can be scaled to any size. It’s made up of stackable layers which provides an ingenious way of turning the contents.
See Mark’s website for detail instructions:
Mark Well’s compost bin and construction instructions
A few additional notes:
Not being that great with wood-working and wanting to avoid the splitting issues Mark mentions in his instructions, I opted for bolts and nuts instead of wood screws. This added $40 of cost but to me it was worth the avoided hassle with splitting wood. I used 1/4" bolts with “locking” nuts (these nuts have a plastic insert that keep them from loosening over time). The bolts are 3” long but this may need to vary depending on the your cut of wood.
Be precise on your measurements for the plank lengths. It’ll payoff when stacking the layers together.
To help with some slop in the measurements, I added a washer between the planks and the corner blocks. This helped immensely when fitting the layers together. If you don’t mind the cost, I would recommend two washers per.
For precise drilling, I created a template (available here as a Visio file or in .SVG format) and used a nail to mark each drill point. You can scale the template to match your size of wood. The corner blocks were over-drilled with a 5/16” bit (for a 1/4" bolt) to allow for slop in drilling so all the pieces fit together.