I think switching to aspen will only absorb the water and keep it in the tub longer... Have you tried like a mini dog bowl? theyre wider at the bottom then they are at the top so its almost impossible for them to tip em over... Heres a pic of what I use... This one is larger than the others I use but works none the less... Check out walmart/target in the pet section... thats where I found mine for like 3$...



As far as your heating problem... are you using the same sized heating pads in all your tubs? If ones bigger than another that tells you why your having issues getting them the same... The bigger one will always heat up fast and youll always have issues... Only put 2 or more heat pads on the same thermo if theyre all the same size... If they are the same size... Could just be the probe placement for your thermometer... None of mine are exactly the same but theyre all within a couple degrees so its fine.