For anyone trying to breed Leopard Geckos successfuly then a reliable incubator is a must, this is from first hand experience. I had a whole heap of problems trying to keep the eggs at the correct tempreture and humidity until i built my first reliable and also relatively cheap incubator. I found a very good online reptile shop and purchased a Habistat Mat Stat Thermostat to maintain a constant temperature, a 25x15cm Heat Mat and a Polysterine Box with internal dimensions slightly larger than the heat mat.
The eggs themselves need to be slightly more than half buried in insulation material such as perlite, sphagnum (peat) moss or vermiculite mine being in the later. The vermiculite according to web sources should be at a 1:1 vermiculite to water ratio allthough I have found this to be too dry and subsequently my eggs began to dehydrate and collapse. If this occurs simply misting the eggs daily will add more moisture causing the eggs to swell back out. I have added water to rehydrate the eggs and they swelled back to normal but I could not quote you as to how much I had to add. To maintain this desired level of humidity the eggs and incubation material have been placed in an air tight tupperware tub with the tub 3/4 full of vermiculite.
The tupperware tub with eggs is then placed inside the polystirene box, to record and maintain an accurate tempreature I have fed through a small thermometer probe which sits along side the eggs (I melted a small hole in the tupperware lid for this, the reason I melted the hole is so when the geckos hatch they don't damage their delecate noses on any sharp bits of plastic) As the plug on the heat mat was not removeable I attached the heat mat into the lid of the poly box and fed the cable through the gap between the box and lid. The thermostat has it's own probe attachment but it doesnt read like a thermometer so it needs calibrating. Once done the thermostat will control the heat mat turning it on and of when required creating a quite steady ambient tempreture of choice. Mine is set to about 82F and fluctuates maybe a degree or so either way.
The Habistat Mat Stat was £21 from Reptile Keeping and heat mat from about £12 and if you can't find a poly box for free you can get one from EBay for £3 or £4. This set up cost less than £40 a fraction the cost and just as good as a shop bought incubator.