Ok here are 4 different incubators that i have built myself over the last couple of years.All work well and some better than others for different species.
I will start with my first polybox method
First go to your local aquatics shop and get a decent size poly box.Mine takes 3 large cadburies tubs which was adequate for corn snake eggs.
I placed a heat mat on the bottom of the tub and then used a couple of old glass runners in there to creat a shelf.
Then i placed the three tubs which were filled to two thirds height with damp vermiculite on the shelf and placed the probe and thermometer.
Then in the lid i cut three holes(one for each tub) and siliconed three cd covers over it to make veiwing windows.
As said this worked very well for corn snakes and hatched out many eggs with a success rate of around 99% incubating at 88 degrees i could expect hatchlings at 49 days.Total cost was around £40 as it is on a mat stat
The second poly box i set up for my wifesbearded dragon eggs.
I again went and got a poly box this time slightly deeper and fixed a heatmat to the lid of the box.
I then filled the poly box to around half full with damp vermiculite and placed the probe.
My wife has since hatched out 4 clutches of bearie eggs with a 100% hatch rate incubating at 86 degrees and hatchlings pipped at 55 days.Total cost around £35 as it is also a mat stat