Building a seed drying cabinet

Has anyone built a small seed drying cabinet with a dehumidifier inside?  e.g. inside a broken fridge or insulated large cupboard.
Me and a few other seed growers are sharing ideas on how best to do this and wondering if there's any issues with having a freestanding desiccant dehumidifier inside a small space?
We're in the UK where autumns are often soggy and it can be hard to dry seed.
There's commercial dehumidifiers which require vents, we're looking at free standing ones that collect the water but none of us have tried having one in a small space, we think desiccant is preferable for low temperatures but concerned it could produce too much heat...
Any one tried this can can teach us from your mistakes or successes?
Thanks

A small computer fan mounted high in the cabinet and the air supply from down low with a 40-60 watt INCANDESCENT light bulb mounted inside near the bottom should supply enough airflow and just a little bit of warmth to knock the humidity down, without becoming a heater in your living space. I'm not sure about catching the water, maybe you are expecting more moisture than I would traditionally expect. If this is done in a living space that is conditioned, the humidity will be pretty low to begin with. If it's done in an unconditioned space, I would suspect that the light bulb will generate enough heat to cut it down inside the cabinet.

My mother-in-law picked up an old incubator for $8 and gave it to us to hatch eggs. It needed rewired, and turned out to be a laboratory incubator from a local hospital chain that sold out. After finding out how to sterilize it like a hospital would, I started testing temp control. When it was dialed in to mostly stay between 80° and 90°F, I started drying some low value seeds. There's a small hole in the bottom that's just open and a bigger one at the top that had a drilled through cork with a glass thermometer installed through it. I unstopped the top hole. It worked decently.

I think basically the air inside warms a little, and rises out the upper hole. As it warms, it's capacity for moisture also rises, and it draws moisture from the seeds. Cooler air rises in through the bottom hole, warms, and the cycle repeats. The whole thing is driven by convection and again it worked decently well.

Later I made two tweaks that made it work even better. One, the glass thermometer being removed, I found a digital indoor/ outdoor thermometer with the outdoor unit being attached by a long wire. I put the display on top with two-sided tape and put about 6" of the outdoor probe in through the hole. Then I put a muffin fan covering the hole. Now it works the same only faster, as the air turn-over is faster. It works great, though it could be larger. I use it less now, since I no longer have help in the garden, and since I found an excaliber style dehydrator at goodwill for $20. At the lowest setting, the heating element doesn't come on, it works great for my needs.

I can find or take some pictures if anyone wants details.

1) Here's how it looks from the outside.

2) Open hole at the bottom.

3) Upper hole and thermometer probe.

4) Interior, muffin fan, thermometer display.

5) 12VDC source for muffin fan.

The muffin fan was labelled as 12VDC. A local thrift store had these wall-warts for $0.25 each. The labels tell the output voltage, so I bought several 12V ones. Works great!

6) Power entry for heating element. Original insulation was ruined, new wire installed.

7) Added wiring: switch controls both the outlet for the wall-wart and power to heating element.

Today I plugged it into my Kill-a-watt for the first time. While heating it seems to draw about 62.2w. Somewhere the incubator is labelled 60w, and I added the muffin fan, so this is about what I expected.

I can say, no I have not built a seed drying box specifically with fan and ext. However I have been harvesting, drying and saving heirloom seeds for over 20 years. I use a simple corner cabinet with shelving. All my seeds get placed on paper towels with cooky dying racks under. I place a white sheet over the glass door to keep sunlight out and that is it. They are allowed to dry over the next 2 to 3 months without interruption. I have never has mold issue or any issue to destroy seed viability. After drying they are placed in paper envelopes ziplock bags to make sure they stay dry and don't take in any moisture and kept in the North side of my 1st floor (nor basement, too damp) in a closet. Basements and refrigerators are too damp.
I have had seeds over 10 years old that germinate just fine. I see no need to bother with such things that rely in things like fans. Set yourself up for success with or without electricity.

Test it as you like with humidity meters or trials of various things, but I've never found or made any dryer more effective and economical than 1. your vehicle parked in the sun, with windows slightly open or not depending (close at night and in cloudy weather)...2. an unused greenhouse in the summer, again vented or not depending....or 3. the attic space of any house.  All can be improved by adding a fan directly on the stuff.  I've successfully dried sliced tomatoes multiple times in these locations in a humid climate...

Alder Burns wrote:1. your vehicle parked in the sun...
2. an unused greenhouse in the summer...
or 3. the attic space of any house...

True, those are effective drying methods. They can be hotter than most folks like to dry seeds. (Common reccomendations tend toward staying below 100°F, but seeds may be tougher than we're usually led to believe.) Worth a try if you're willing to monitor temps and/ or risk a few seeds. (Testing my rig or any other involves the same risk as testing your suggestions.)