blink - "Crawl space" would indicate that you are growing under a house - is that correct?

As far as the construction of your box goes - with what you hve in mind you have reduced the interior height of your box to about 3.5', maybe less. Build the box with 1"x2"s or 2"x2"s and do away with the styrofoam inside. Also, don't use spray foam on the corners - it's not made for that, it doesn't work well for that, it's hard to use and... it's just the wrong stuff. Use caulking instead.

If your crawl space is under a house, be sure to use GFCI breakers for all of your electric.