Those small, low-cost incubators are more for a one-time project. Inconvenient and not at all durable. Ideally, they should be well-sealed to maintain both temperature and humidity. I justify this expense by asking myself 'How much money do I lose if the incubator kills a clutch?" "Two clutches?" "More?" I had three clutches of eggs in one incubator at one time - I felt remiss "only" spending $180USD on my first one, not to mention having only one. "All your eggs in one basket..." as the proverb goes.
I used a bar-top wine chiller to make a great incubator. I disconnected the solid-state heat exchanger (to defeat the cooling function) and the exterior 'condenser' fan, but left the interior circulator fan running. I also left the thermometer readout and interior light working. I have electrical and electronics experience, so working out these changes wasn't too hard to accomplish. One could skip the mods, just leave the chiller unplugged and still have it work fine, but at least a battery-powered circulator fan should be used inside to keep the temperature consistent throughout the compartment.
Once finished, the wine chiller incubator system held 85F~29C within 1 degree and the entire interior space is the same temperature throughout; within one degree of 85F, and most often, 85 exactly. It was monotonous, set point 85, probe temp 85, analog thermometers placed at various areas inside show 85 too.
https://www.aussiepythons.com/threads/wine-fridge-incubator.228594/#post-2543840
The thermostat is the heart of the operation and everything depends on being able to control and hold temperature without excessive deviations. Buying cheap here is to gamble with success. I bought a 'Vivarium Electronics' VE-300x2 thermostat. It is a very expensive dual-channel unit, but has excellent control and flexibility. I could have done fine with either of the next three steps down (VE-300, VE-200D, VE-200) but no one had any of them in stock and I had a time problem with 17 monitor eggs at stake.
As 'herpetology' says above, the single biggest thing is to have it "dimming" (pulse proportional control), so you always have just enough gentle heat to keep temperature where it needs to be, instead of having a simple 'on/off' function that is either 'off' or 'full blast' and bounces internal temperatures all around the set point temperature. I'm in the US and I don't know what is available where you are, but I am very pleased with the Vivarium Electronics product and their customer service.
I used two 8 watt under-tank heat mats that were always slightly warm (never cool, never hot) due to the pulse-proportional control. I left the wine chiller's internal circulator fan running and that was plenty to keep the inside air well homogenized. I first tested one 8 watt matt alone and running full power, full time, it brought the temperature up to 80F~27C, with ambient room temp at around 65F~18C. Two 8 watt matts were more than enough and would have easily held it at a much higher temperature.
A commercial incubator I initially bought for $180USD had several minor issues before it suddenly failed:
1) There was a temperature offset between the set point and the probe reading, so I had to offset the setting to get it to hold target temperature...according to the probe. All digital, calibration not possible.
2) Neither the set point nor the probe temperature were correct. I has to rely on analog thermometers (two calibrated 'core thermometers') to find and set the actual temperature.
3) Temperature fluctuated a few degrees. It would let the contents cool down two degrees below set point, the start and run until it was two degrees above set point and shut off. Maybe not critical, but likely not optimum.
4) Temperatures throughout the incubator varied by several degrees, both vertically and front-to-back. I had to rotate egg boxes to try to ensure equivalent incubation conditions.
I selected that model because it had good reviews/comments and because it would cool as well as heat. That cool function was important because I had to keep it in a second floor space that got very warm in summer.
Then, the commercial incubator failed in the middle of the night, tried to pull down to 40F~4C and almost killed a 17 egg clutch of Ackies.
I had considered making my own incubator at first, but thought it better to trust a professional outfit, as do you. I now regret not having just built my own to begin with.
A few pics:
