Thank you for your replies and sharing your experiences . I appreciate it!
Glass would be probably best choice for heat retention. I believe Grex has glass jars - but only as side gravity option siphon attachments - so does not work.
I was even trying to wrap the cup with the aluminum foil and put a rubber band around to keep the foil in place.. lol. need to keep experimenting on this ). I also keep the cups (I have few) warm as well, so it works for the first "go around"
So.. will stick to a routine of reheat a go but without time consuming rechecking temperature in the cup every time (as this is what I have been doing, worrying not to get CB too cold).