I think the best way to do a rat bin like that is to leave extra on all sides of the wire mesh. and leave the extra so its not cut right at a square but so that the wire is poking out on all the edges. I can't think of how to describe this. Let me see if I can make a picture.

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You want it so the edges of it to look like that. Then what you do is bend those extra wires 90 degrees. This way you will have a mesh grid would be raised off of the table if you set it down. Then what you do is heat up those edges so they are very hot. You then push those edges through the tub and since they are hot they will melt right through. Once through you can finish by bending the other 90 degrees so it locks them in place. I wish I had some pictures of this to show. It keeps the wire mesh on the inside and the sharp edges on the outside and doesn't allow the rats anywhere to chew.