Do you have the option of building something? I'm in the process of building what are basically referred to by the ATF as "Type 1 Magazines", which meet my insurance requirements. Basically they will be 6x6x8 wood boxes with 1 inch thick wood siding and flooring. Framed inside with insulation. Door will be weather sealed. No metal in construction other than hinges/latches. They will be on skids to raise off the ground (concrete in my case), but the skids in other circumstances would mean no permit required (not relevant to me, may be relevant to you). Anyway, they will have a small 6K BTU window AC unit mounted on them, and a small dehumidifier (plumbed to a drain so it doesn't have to be emptied). I use a brand of wifi monitor made by Govee that reports temp and humidity to my phone, and will have one of those in there and a wifi smoke detector. It will be relatively cheap to build, especiallly if you scrounge stuff. Mine will be under a cover, but wouldn't be much extra to add tar paper and Kool Coat to the roof, along with a gutter all the way around, or build roof angled with slight front overhang, and gutter on the rear. And you would obviously not need to meet the 1 inch requirement for wood siding, so could go super cheap there, like 1/4 or paneling even if it isn't going to be somewhere horribly exposed to bad weather.