hi i tried to make my own incy out of polystyrene, a light fitting and two bowls of water, a bit of tape, a glass off a photo frame. the humidity was perfect and so was the temp for a while but then it kept rising so will have to do fase two, have got to get hold of a thermostat. but it prooves that it dosnt have to cost alot.
The big problem with such incubators is making them sterile for the next time you want to hatch. Polystyrene is very difficult if not impossible to sterilise well. Hatching is very messy and produces a lot of dander fluff and pathegens which needs to be removed completely after the hatch.
Maintaining a stable temp is also difficult and using a way to stir the air with a LOW speed fan to prevent hot spots is a good idea as well as maintaining some ventilation and as stable and moderately accurate humidity as possible. A high airflow is not good either as too much will cause uneven drying if the top of the egg is in a draft but the bottom is in still air.
The heater should not be too hot and a large area lower temperature heater is better than a single higher temperature small area heater. Radiant heaters if used directly are likely to cause temperature gradients across the egg and aren't as good as large area conductive or air space heaters. One way to avoid this might be to heat a large area alluminium plate pianted black 6 inches or so above the eggs rather than use direct radiant heat from a lamp.
During the first 18 days average humidity needs to be around 40 to 50% RH to produce the right water loss in the egg. Turning the egg is critical during this period to prevent any drying of the egg air interface inside the egg. Accurate humidity measurements are difficult and the sort of cheap humidity sensors sold on ebay are not very reliable or accurate. I'd be very cautious about relying on them though precise humidity isn't so critical. If the humidity is as noticable as you suggest I would be concerned it is too humid. For a home made incubator I'd calibrate it by using the egg weight method. Use a couple of 'sensor' eggs in fixed positions and regularly measure their weight loss (you need an accurate scale) and plot a graph against time. The % weight loss you need to achieve can be obtained from charts. This is a direct measure of what in fact you are trying to achieve and is better than just measuring humidity. Too much weight loss indicates too low a humidity and vice versa. I'm affraid it's a trial and error calibbration method but one which commercial incubator manufactuers will have used to calibrate their incubators and produce their instructions.
The thermometer needs to be more accurate (within half a degree or so of an optimum 37.5degC. A way of ensuring even temperature for example by using a low speed fan is very important too.
There's no reason you can't design and build your own incubator but it's very likely they won't produce as consistent or a successful results as good commercial ones such as Brinsea and RCom.
HF