Hi,
There are alot of papers on the internet that will instruct you on how to incubate reptile eggs, and there is almost no reason why viable eggs shouldnt hatch. Adam, your right, it only looks as if there are 3 or 4 viable eggs in that lot, but how long did it take for these good looking eggs to discolour and cave in? There is still the posabillity they were not viable at all. If they went bad within a few days to a week, I would say they were not viable and you possibly did nothing wrong.
There is no reason why flys would be a problem unless the eggs were bad in the first place. The egg shell is there to protect the egg from such things and a healthy eggs is unlikely to be fly blown.
You can built an incubator any way you like. The important things are simple.
HUMIDITY- 50% vermiculite 50% water by weight ( or a little less water) will provide the correct humidity. Place this in an airtight (or near airtight) container and half or completely burry the eggs. I would use a 20lt container for say a dozen eggs, and half fill it with the vermiculite mix.
TEMP, the temp INSIDE the egg container is whats important... it should be between 28 and 32 C
How you achieve this isnt important, as long as it is constant. You can use a couple of light globes and a good thermostat and place the probe inside the egg container through a small hole. a well insulated incubator is best, I have seen people use a foam box, esky, and one of the best is an old fridge.
Dont take my word for it, goto
http://www.smuggled.com/egg1.htm and read Brian Barnetts paper on egg incubation.... Its simple, dont question his method, because it has worked for many hundreds of his snakes, and probably thousands australia wide. It will answer all your questions.
regards
Craig