Plastic box with hard but not too brittle plastic, with a lid without locking. Dimensions correspond, there are no unnecessary obstacles during installation. I use it for various devices, such as a transformer with variable power.
Pretty good ABS project box. Mine came with 4 inside plastic standoffs to fix the arduino board. The back snaps in place and it locks the box very tight. You need to use a blade to open it up again. It is a good box for arduino mini size projects.