Absolutely! It is exactly what they are designed for!
I have a two tier hutch and use four snuggle safes for my two boys. I put two upstairs (one in each of their huts in the bedroom area) and two downstairs (one by their hay rack and one under their pile of hay that they tunnel and play in). In the bedroom huts, I just put them in, sometimes I put them under hay but not always, they still stay warm the same length of time regardless. They come in a fleece cover so piling hay on top of them isn’t entirely necessary.
i pop them in when it is 12 degrees or less, day or night.