As promised here are some details of my home built proving box. Some may call it proofing - but each to their own.
I use this for temperature controlled storage of bread and pizza doughs either in bulk proof stage or for shaped dough balls or loaves. It comes in very useful in deepest Lancashire - I have never felt the need for a cooling option!
I decided to make the box "collapsible", otherwise it would take up too much precious storage space, so there are hinged pairs of sides which join together with magnetic catches. The panels are a sandwich construction with ply inner and outer and polystyrene insulation in-between.
Heating is by a 240v AC cartridge heater set in a heat sink, with a 12v DC fan providing airflow, low speed all the time except when the heater is energised, when it runs on high speed.
Temperature control is by a cheap on/off controller off the 'bay, which does the job well. I usually prove in the range 22-25C.