I've actally done a lot of 'looking into' of incubators, but have never used a homemade one, so feel free to use or dismiss anything i say as you see fit
The only incubator i have used is called a 'hovobator', i used that in the states, but i didnt really like it. the temperature seemed to fluctuate more than it was supposed to/advertised.
I'll build one myself for next time - this is a rough outline on what i've decided after a lot of study/thought
As the main material for the incubator i'll use MDF (medium-density fibreboard) for a couple reasons.. 1. you can screw into the end of it (though the hole needs to be predrilled to a decent size - it can split) so no need for a frame. 2. it comes in a few thicknesses - more insulation. 3. - there's a reasonable chance i can get it for free 8)
I'll also probably line the inside of it with styrofoam sheets, as there's nowhere in my house with a reasonably constant temperature.
Door will be on the front, and probably hinged at the top.
The size will depend on how many containers i will have inside it. depth will be the same as the depth of the containers, height enough that the heat source is a fair distance away (1' or so).
Heat tape seems like it would work well, or several low wattage light bulbs.
Probe thermostat, and a probe thermometer or 2 to monitor. insert the probes into the actual egg containers - go off the temp reading on the thermometers, not the thermostat. also another thermostat set at a couple degrees higher, j/i/c... seems like a plan.
half burying the eggs in moistened large grade vermiculite seems to be the consensus for most species - worked for me the couple times i've used it.
ehm.... that seems all the important stuff.. a lot of information can be found in books or on the internet, look around. that's where a lot of my ideas were spawned, sorry for not giving credit, but i've forgot where i've read stuff.
oh also my reasoning behind building my own is this - buying a proper one is sooo expensive heh
anything i forgot to say, remind me. right now though, i'm hungry