Gunilla gave you a very good design. I do make a couple of modifications that I find useful.
1. I add a small aquaium pump either air or water to circulate the water in the main incubation box. I find that with circulation or any movement of the water the temps stay more equal throughout.
2. I put the lids on my egg boxes and then the lid on the sweater box. This saves the added expense of the tilted plexi, the egg box lid keeps the condensation from dripping on the eggs.
3.I use perlite for an egg incubation medium. It's coarser granules allow for better air circulation around the eggs. Fewer moldy eggs and easier to keep the humidity levels right. Mix 1 part perlite to 1 part water if you have a scale. Or 1 part water to 10 parts perlite if you use a measuring cup.
Examples. By weight, 200 grams of perlite to 200 grams of water. Use whatever weight system you're used to.
By volume, 2cups water to 20 cups perlite.
Keep and aquarium thermometer in the egg box, adjust the temp on your submersible heater till the desired temp is maintained in the egg box. It takes some time to get this calibrated so don't delay till the last minute.
Hope this helps
BigT