Hi Nazza,
Well your english is very good for a third language. Good enough that I hope you and I can stay in touch and discuss Gonyosoma. I used to speak Spanish well but I have forgotten most of it.
I've lost four clutches of Gonyosoma eggs myself so hopefully we can figure it out.
I don't want to discourage you from using a hydrostat but I'm not sure how accurate and effective it will be. Hydrostats available in the US are not very accurate at all, especially as hard water deposits build up on the sensor. Maybe you have better technology available to you where you are.
For what it is worth, it seems like some of the most successful breeders of Gonyosoma use very simple incubators.
Like I said on the ratsnake forum, a friend of mine recently had his best hatch rate ever by incubating his eggs in a plastic box on the floor of his reptile room. This was after many attempts at incubating them in an expensive incubator.
Regardless, I like the idea of using a fan that can quickly remove the air in your incubator. If you leave several large bottles of water (sealed) in your incubator this will provide a lot of thermal mass and you should not see significant temperature drops.