First off heat pads do nothing for cool side temperatures. They only heat surfaces not the air in the cage (which is equally important)

You can try to insulate the tub. I've even seen a few people suspend a heat lamp (50-75 watt infrared bulb) above a single tub to help raise the air temps. If you can't raise the air temp you might need to switch cage types. A PVC reptile cage or even a glass tank will be better if you can provide proper temps and humidity. This is the biggest downfall of heat pads and tubs that nobody seems to mention when they claim that tubs are the best way to house BP's.