Sphagnum moss will work yes. The medium yes will be to big for an 85f snake but you can stuff the one side until the snake has a tight fit. You can buy the smalls so you don't have to stuff the hides but you will have to buy mediums in a couple months so either pick up both sizes or just stuff with moss, it's up to you.
Swell is a good supplier I hear and I was going to post a link but they were out of the mediums.
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