Why not use a Raspberry Pi or similar? Cheap, small, and you don't have to worry about wifi.
Well there are a couple downsides to the pi, they are more expensive, you're probably going to have to order one, they need an sd card (which they eat), they need a bit of knowledge (I am talking about easy cold storage for everyone) , they need a tv or monitor.
I think the wifi is not an issue but that is me personally, if it is disabled and stays in a signal blocker bag i don't see how it could compromise security. (Totally open to reasons why)
I like the idea with the bag. Any faraday cage would work, e.g. the right box. I will see if I can get an old phone and test that. I wanted to setup and old laptop, but this would be way easier. At least I think so.