I have a chest freezer that I operate at 60F/15C and brew all my beers inside of it. I also use a raspberry pi system that I keep inside the chest freezer. What can I do to prevent moisture growth inside of the chest freezer?
I'm afraid there is no simple answer. Some refrigerators are made in a way that prevent moisture and frost. it generally consist of wall coating that makes water flow easier, and draining ducts. And also periodic heating of cooling elements to melt frost. If your refrigerator is not built with these, your options are limited:
Buy anti-frost freezer mat. They are not as miraculous as manufacturers like to claim, but are not bad either.
Use anti-frost / anti-fog coating, to let moisture flow to the bottom easier.
Keep your raspberry pi system outside instead.
These are not exclusive. Of course you can, and maybe should, use all of them.
A good humidity control for a fermentor is a mason jar of rice. Air circulation helps a lot to facilitate contact with the rice.
Change it out when the rice doesn't appear dry anymore.
Pillow packs are good too. Silica moisture packs are a must have in homebrewing, for grain storage too. I save them from sneakers, pillows etc. They are also reusable, once damp they can be dried back out in the sun.
I use an Eva Dry 'rechargeable' dehumidifier (https://www.amazon.com/Improved-Eva-dry-E-500-Renewable-Dehumidifier/dp/B00BD0FN8A/ref=sr_1_14?ie=UTF8&qid=1475687798&sr=8-14&keywords=dehumidifier+mini) and it works wonders in my kegerator.