For extra under floor support in a crawl space, what I probably would look into first is one or maybe two simple adjustable screw floor jacks in the appropriate length for the height of the crawl space. They are relatively inexpensive, available in assorted adjustment heights, and can be found at just about any home supply store such as Lowe's, Home Depot, etc.

Installed they would look something like this except with just one or two jacks:

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