RE:
Random thoughts, some based on decades of experience in the scientific world, some total WAG!
I've been using SwitchBot IP65 Indoor Outdoor Hygrometer Thermometer Wireless 3Packs and they seem to work well. Accuracy is good compared to several other references I've got. They can data log for 68 days before rollover and I can check the temp and humidity graph on my phone Bluetooth anytime I feel the need.
I think most people's silica gel desiccant isn't as dry as they think. You have to bake it in an oven just below where the envelopes are damaged to drive the water out. I thought it would be nice to dry it in a filament dryer, but a recent video showed that was near worthless. They also used a microwave, but that method seems labor intensive. I use a small oven. In another life I used 4A Zeolite as a lab desiccant and the stuff will work wonders at low RH, but you have to bake it seriously hot, about 300C for several hours, to get it dry, and then prevent readsorption while it cools. At that level of dryness, you should avoid skin contact.
Inexpensive consumer dehumidifiers are crap. If they last two years you're doing well. Maybe 3-4 if you get a good one, but there's no way to tell if it's a good one. I use one in my hobby room and it will keep the room at about 45% RH during the summer. In the winter, without it, the room runs about 36% RH.
You probably can't build an airtight box. Well, maybe if you're a welder, have a lot of stainless-steel plate and access to a Veeco helium leak detector. The best box you can build will breathe as the barometric pressure rises and falls. And you certainly have to open it every now and then. That means using a decently dried desiccant and changing it out on a regular basic, based on a humidity sensor. It's a process that you just have to keep up with.
Too soon for a review, but I recently got the Creality SpacePi X4 and it seems to work quite well.
RE: Humidity Control For Filament Storage - How Tight Does a Container Have to Be?
In my experience, that is low enough for 99% of filament I use.
Except for PA6 and PPA, I agree.