I do some dodgy stuff, but that just looks a fire hazard.
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.
The plastic will not melt until the oil heats up to close to the melting point of the plastic. Don't believe me? Try this: put water in a paper cup and hold a lighter to the bottom. It may blacken a little, but it won't burn. You can also put water in a plastic bottle and put it in a campfire. If you leave the lid off the water will have to boil off before it will melt. (If you leave the lid on, congratulations you have made a steam bomb... run unless you like hot coals flying at you.) If there is enough cardboard to heat up the oil, when the plastic does melt the oil will easily ignite, making your problem much worse.
If you're trolling, good work. If not, please take the jug off your rig.