I have changed a couple of things that are not shown, but you can see all the basic features. Outside dimensions of the cabinet are 38"L x 38"H x 24"D - or, since I think S. Africa uses metric - about 1m L x 1m H x .65m D
Initially I used an old waterbed mattress heating pad, but it was nowhere near as accurate as I wanted. I went to the porcelain socket with an incandescent bulb as a heat source. I first used a 100w bulb, and frankly had to do quite a lot of fine tuning to keep the temperature in an acceptable range through the heating up/cooling down phases. You need to be as close as possible to 99.5 F, and the less variation, the better. I managed to get a successful, healthy hatch, though.
Just a day or two later, the 100w bulb burned out. I replaced it with my backup - a Phillips 75w true daylight soft white bulb - and tweaked the (water heater or geyser as you call it...) lower element thermostat a couple of times.
Magic! A constant temp with limited variations staying between 98.6 F and 100.3 F!
I currently have 23 eggs in it, though the top left drawer would hold close to 50 if I arranged them more tightly. I could also add at least another three such incubating/hatching drawers/shelves to the cabinet, just on the left side of the partition alone.
Lightfoote