I had made a box before out of one of these home depot like boxes, but I made the mistake of cutting down entire front edge of the box.
My fuzzy diggers decided to throw pellets and stuff all over through the cut down side.

After a short trip to depot again to get a new box I cut down only the middle to allow allow them to get in/out.
This seems to have worked and was approved by both fuzzbuts after some inspection.
The box fits perfectly in the width of the cage and is 190mm tall so not much chance of digging causing cleanup work.

I cut up some food grade pvc tubing and threaded it onto the cut edge to avoid any sharp edge and hopefully limit the gnawing a bit.
I secured the tubing with some zipties and heated the cut off ends of the zipties with a lighter to remove the sharp end after cut.

Peter is buisy and not too happy about me taking a picture of him while he is “working” but it was important for illustration purposes…