Hi Burgalicious,
Any of the less expensive foundations in the foundation thread here will be fine. The wire grid foundations are good if you want a frame / foundation combination with some storage space under the bed and the KD foundations will also work well. Most of the KD wooden foundations have gaps of a little over 4" (which should be fine with the polyfoam base in your mattress) and the nature’s sleep has gaps just over 3" (which is more evenly supportive) and the metal framed KD has the most slats and would be the strongest of all the KD types.
It should probably expand most of the way in an hour or two (mostly much faster than that) and may expand a bit more over the next 24 - 48 hours.
That’s normal when it’s been in plastic during shipping and it will normally dissipate quite quickly (as it seems to have done). It will be fine to sleep on as long as the odor is mostly gone and doesn’t bother you.
I’m looking forward to your “report” after you’ve slept on it for a bit 
Phoenix