Sorry to hear of the dilemma with cake. Why don't you try placing him on paper towels for now and until you can monitor the conditions progress. Also the tub that he is in, does it have makeshift ventilation holes in it? Are the ends and points of the holes facing into the tub? If so those points can be pretty sharp and may require reheating and smoothing from the inside out. The brown dots can be the particulate matter in the aspen. Aspen has a drying quality to it as well. You may need to consider adding a handful of 100% cypress mulch to the interior of the hide. What temp reading do you maintain at the hot spot?