Waterproof enclosure from Amazon with cable glands on everything and hope for the best. A good power supply (Meanwell or almost any knockoff) will have a thermal cut-out that will kick in before the box melts, but oversize it a by a few inches. At -30 / sub-freezing the rest of the time you won't have an issue.
I'd add a temp sensor because I rarely even trust myself. But the spec sheet should provide some additional comfort, the last pages are usually graphs of permissible load for a given temperature.