This maybe an alternative to your need.
In most ordinary electronic-components shop there are several bi-metalic thermo-switches of different temperature level. The sizes are about the same with a power transistor 2N3055, also with two holes for the screws. What I have in my drawer are that...
The variable resistor in other system does not directly control the needle. Instead, in controls a bimetalic- mechanism which in turn averages the reading within certain period of time. This produces that slow response.
SUNAN
