Close in the bottom?
I closed in the bottom of my cabin with PT plywood on studwalls and insuated that. Whether you can do this easily/inexpensively or not depends on the grade and how high up on piers the house is built.A side benefit is that any pipes in this space are also kept warmer. Depending upon the moisture content of your soil, you may need to put down plastic as a vapor barrier, as well.
With a lockable hatch - it makes great storage space for tools and toys, too.
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