Standard Mechanical Thermostat
This classic design will fit most spas. The temperature probe (sensor) is 3-3/4" long by 1/4" thick and on a 36 inch malleable coil. If you don't require the entire length, simply leave it partially wound up.
The thermostat probe should fit completely inside the thermal well. It is OK to replace with a smaller thermostat probe, but not a larger one. If it protrudes outside the thermal well, it should be insulated.
An easy way to insulate the back of the thermal probe is to cut a slit in a Styrofoam cup. Then fill with spray foam insulation and cover the rear of the hot tub thermostat probe.