@loganfive Material wise, I would always suggest a plastic material such as ABS or polypropylene as these will give you the RF transparency you will require. Anything plated or die cast might look the rugged part but would more than likley affect your antenna range.
If its going to be situated on an external door then look at an IP67 rated enclosure too as it'll give you the waterproofing (you mentioned the cold so moisture won't be your friend!) you require.
Iinitially when I read your post I looked at external rfid enclosures to see if an existing one could be pressed into service. Unfortunately most of them come as pre-assembled units that are epoxy or silicone filled and would be a swine to work with.
A quick search of aliexpress however shows quite a few that look like they would work :