We use a big stand up freezer. We made stackable wire racks about an inch deep. I mounted a 12 volt computer fan powered by a plug in power supply to blow the air around inside. Then we place a new bag of kitty litter in the bottom with a bunch of holes punched in it.
Freeze drying is the same as freezer burn. So the kitty litter will absorb the moisture from the air being circulated. But depending on the food type, it will subliminate to freeze dried within 3 weeks, most foods cut thing in 2 weeks.
But the same can be done in smaller batches in smaller freezers also. The key is some kitty litter to help draw the moisture to the surface of the foods to subliminate. The ancient Inca used to freeze dry foods in the high altitudes of the Andes the same way. :)
Any particular kind of kitty litter?