I have a few of the Bee Forever boxes, I like them very well, I run SBB on my hives so I donâ€™t worry about the insulation values of the box, seems like there is a lot of beekâ€™s running SBB all winter long with good results. So with the bottom wide open the temp inside is not going to have much to do with the insulation value of the box. I have seen only one study showing wrapping the hive and the results were not conclusive either way.
Regarding plastic emitting chemicals, Polypropylene is a very stable material and does not contain PVC like vinyl does, PP is used in Food containers, Should be as safe or safer than a pine board. If used with care they should last a long time. PP is also good for impact resistance; it tends to deflect a blow rather than a more brittle material that would crack.