The cold spots are in relation to the screw holes, not that surprising. Thank you for doing this. I myself am about to do a reading with a silicone thermal pad on the bottom of the glass.
bust as in didn’t hold it in place, or bust as in not-level?
I’ve been considering a magnet-backed glass sheet for the usable portion of the bed (about an inch in from the corners), to have the flatness of glass to print on but using the clearly warped edges of the bed as a reference surface (fallback: MDF under the bed, full-sized glass on top).
EDIT: Scratched out Melamine. 250C decomposition temperature just doesn’t sounds like a good idea, though the bed doesn’t get that hot.