The best way and perhaps the only way to keep rats out of the compost bin is to raise the floor off the ground so that they cannot burrow into it.

This type of bin is also easily knocked down into the sides, bottom and front boards.

Here is a link that might be useful: Compost Bin Construction

I find the piles of tailings that the rats leave outside the bin to be quite convenient; just shovel it into buckets and take to the garden.

Thanks for suggestion cowgirl2, & I do like your design. Reason I didn't make raised bins is that our English rats like to nest under things like that... if I add another two bins they'll have a perfect spot to bed down with the warmth above. But you did get me thinking further & I've now surrounded the bins with paving slabs, see updated blog post at link below.

Here is a link that might be useful: An English Allotment Garden