You could hire a residential insulation company to spray the entire underbelly with foam. I see these guys spraying during new construction and I am amazed at how good they are at controlling the spray to allow for the perfect amount of expansion. Any overspray is very simply rasped down.
You would want to spend some time on thought and preparation of areas that you might not want to spray or draw a nice diagram with good measurements of where certain things are in case you need to cut through the foam for any reason in the future.
Bruce