I put a motor in my dryer. I used a N20 geared motor (Brand: Greensen, 68rpm at 12v) and two Ikea AAA battery’s ~2.8v and the result speed for the spool was ~14 rpm.

Lid opening ~40% of the area compared to having the lid partially open (standing on locking mechanism). I reduced the opening because the device struggled to reach set temperature in the filament.

The built-in temperature and humidity gauges are useless so I used other ones.

The built-in humidity sensor is probably located behind the display and temperature is probably just an offset from temperature sensor on the heating element.

Test:

Set temperature: 50C
Filament: 1Kg black ABS

0h:
Humidity 49% (round hygrometer meter)
Filament: 26.9C (IR-meter)
Top air temp: 26.7C
Motor temp: 26.3C (IR-meter)

1h:
Humidity 34% (round hygrometer meter)
Filament: 40.3C (IR-meter)
Top air temp: 38.5C
Motor temp: N/A (IR-meter)

3h:
Humidity 29% (round hygrometer meter)
Filament: 42.3C (IR-meter)
Top air temp: 39.3C
Motor temp: 35.6C (IR-meter)

6h:
Humidity 26% (round hygrometer meter)
Filament: 43.2C (IR-meter)
Top air temp: 39.1C
Motor temp: 35.7C (IR-meter)

The geared motor is almost quiet at this low voltage/rpm but it’s amplified by the box so it’s a little noisy. This cheap gearbox isn’t the best and I don’t know how long the worm gear will last.

I believe that it safe to use 50C setting with PLA if the spool rotates all the time.

What you need:

N20 geared motor (68rpm at 12v) with two shafts
https://www.amazon.se/-/en/gp/product/B0D181F8H3/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1

Rubber tube with inside diameter 5mm and outside diameter 7.5mm (I used Car Windshield Washer Hose)
https://www.biltema.se/bil---mc/bilreservdelar/torkarutrustning/vindrutespolare/spolarslang-5-m-2000031910

2x M3 screws

STL, STEP and Fusion files included.

Note:

I did a retraction tower before and after drying the filament at 50C 6h and there was no difference for PLA. The PLA was delivered in a bag with desiccant but the air inside the bag was ~ 40%. It was also left out in the open for about a week. I don’t think a filament dryer is needed for PLA unless you live in a very humid area.