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If you have access to a bench or circular saw, I should try and close the gaps between the 'planks' of the vertical sides. You can always dig out the compost, turn it and replace with the occational spread of chicken muck to help it break down. Cheers, Tony.
If you have any spare 1" X 1" wood strip, nail it up both the inner faces of the bin leaving a gap between sufficient to take the planks prised from another sacrificial pallet. As your heap grows you can drop these planks in and remove them to access your compost.Cheers, Tony.
The compost will rot down so you keep topping up, the compost will solidify (become firmer) and should remain stable when you lift out the planks. Cheers, Tony.