everyone needs to learn some more physics.
if it maxes out at 91 on a tabletop, it will max out much higher if there is some insulation added. like a tub on top of it.
and if you would insulate t even better, like putting it between two layers of styrofoam or even aerogel, you would get it to melt and/or burn.
its really basic. you have a constant energy output in the form of heat, so the maximum temperature you reach will be entirely dependent on how fast the heat dissipates away.
adding more aspen shavings is going in the wrong direction. this stuff insulates, so underneath the aspen you will reach even higher temperatures, making the situation even more dangerous. the snake might dig towards the heat and get burned (this actually happens). a better strategy would be to make it easier for the heat to dissipate: add an air gap underneath the tub, and place the water bowl directly on the hot spot. but i suggest a more drastic solution: just switch it off until you get a dimmer or thermostat, and make sure the room does not get too cold. that really avoids the risk of a burned snake and property damage. it will be fine for a few days
the correct solution is a thermostat, or at least a dimmer. with a dimmer its still not actively regulated and will change depending on thermal insulation and ambient temperature. but at least with a dimmer you can cut down the output. a thermostat actively regulates the temperature, it is the optimal solution.