I used the water heater thermostat (lower, electronic water heater thermostat) in my home made bator. BUT, it still varied
a lot during the course of the day/night! I baby sat the thing, got to know the time of day it would start changing (like 3 pm to 6 pm it was hottest in the house, and would peak in the incubator) 5-7 AM was when the temperature would drop. I litterally got up several times a night to check on them and adjust. I did this the entire 3 weeks! The only way to avoid this is to have a room where the temperature is constant, but we live in paper houses here, and it is just one small membrane between the outside and inside, LOL.
The way that electric water heater thermostat works (by the way, it isn't electric itself, it's mechanical, but made for electric water heaters) When the thermostat gets hot enough, it breaks the electrical connection (the negative line) to the light bulb, which turns the light bulb off. Unfortunately, it isn't that sensitive at lower heat, and so it would turn it'self off at say 100 degrees, then turn the light back on at 86 degrees, so obviously that didn't work. Where it became much more useful, was to set it to something much higher, like say 125 degrees on it's dial, and have the hole in the back (opposite side of the dial) right on the bulb (now my box is small, and I used a 40 watt candelabra bulb). This way, you only get about 0.2 - 0.4 degree variation. But again, it's not
digital, just mechanical, it's not really reading a temperature, just physically reacting to heat. So that means You need to adjust it, and fiddle with your incubator until it's functioning in a dependable, though imperfect way. Knowing what temperature you get throughout the day, you can adjust things two ways.
1. Open and close vent holes
2. you can pull the thermostat away from the bulb (have it hanging inside the 'bator and I tug on it's split chord to adjust how far away) to create more heat, but I only do that while keeping an eye on the temperature as it will climb very quickly even when you're only pulling it from the bulb 1mm.
If you don't have a VERY stable temperature room to do this in, then really, you should get a nice dependable incubator that you can set and let it do it's thing

I had a lot of fun hatching, and am a stay at home mom, so I could sneak naps here and there while keeping an eye on my eggs 24/7. It was a lot of work, and I don't want to do it again so much, LOL, but I will be doing it at least 2 more times because I want to hatch out 2 other groups of eggs in the near future.
Good luck! Hope you find something that works for you. Also, I wouldn't throw out your eggs yet, until you see signs that they've died. Just 'cause they were exposed to 115 degree heat, doesn't mean they got over 103 degrees, and they could still be ok (depending on how long they were exposed, if you have a still air incubator, it may not have been that hot where they were sitting, many factors go into this.
Here is the way I made my incubator:
only I put my water bowls under the light (actually on either side of the light) and I installed a computer fan as well as the thermostat.
And here is one guy with a 4 part tutorial that is pretty detailed. My bator was way smaller, though! I could hatch up to 15 eggs in it though. I took pictures of it and all, and I may make my own version of a tutorial? But really, the info is already out there to be found