To answer your first question I think the rack would work fine depending on the size of your rainbow. For a baby a rubbermaid shoebox would be much better. For a full grown adult it would be a little on the small side but workable for a while. Keep the humidity levels high.
My other thought was if you can actually find someone who would trade a rainbow for a BP then you have truly scored. I do shows frequently and rainbows are great selling snakes I personally wouldn't think too many people would be willing to take that offer. Good luck!
Mike
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