I too started making my own incubator and found that despite following the instructions from about 20 youtube videos along with research from just about every site on google, I almost set the house on fire!
The inside of the polystyrene box I was using melted and my room was filled with noxious fumes. The bulb was the recomended distance from the sides and the lid and the wattage of the bulb was 60w.
I know it seems like a cheap option to make your own and many people appear to have no problems but the cheaper option may be to buy a cheaper purpose built incubator rather than a new house.
Sorry, don't mean to be a pessimist but I am just advising from my own experiences
best wishes
Lisa