I have taken some cost effective precautions to avoid a fire.  I filled a milk jug with used cooking oil, and it rests just on top of the box.  If the heat causes the box to ignite, it will melt the bottom of the platic jug and extinguish the flames in a non-conductive coating of fry oil.

Because this cooking oil is used, it provides a few benefits.  1) It is nice for the environment.  2) It's free materials.  3) Even though your mining rig is on fire, you can't deny how much it smells like french fries (and that's a great smell).

The only problem I can see here is the false positive that a smoke alarm would play, so I have disabled that to prevent any unnecessary panic.