Look for a Devtec heater, it has a long u-shaped heating element attached to a small controller box. It's totally submersible. Used them before when I processed Tmax 3200 in Tmax dev at 100°F. You just need to calibrate it to the temp you need and if this is the only setting you see using, drop some hot glue or sealant to keep the knob from moving. I have one I planned to use in my sink but it doesn't go down to 70°.