Not sure what species you are dealing with and we are pretty small time - but this is our set up from this year - working well so far - cornsnanke eggs pipping on schedule -
One of two coolers showing cord from aquarium heater (the lid closes on this brand of cooler without compressing the cord, so no need to modify the cooler).
View inside - some plastic containers filled with gravel to support a plastic rack (dropped ceiling grate cut to size - same as we use in the egg containers) above the water level (1/3 of cooler with water - enough volume for good heat sink effect - gravel also holds heat when cooler opened).
Ebo Jager submersible aquarium heater (they say not to immerse but you can) - set up 3-4 days in advance - the thermostat in these is pretty reliable and we don't use an additional one.
Egg tubs set on grate - each contains moistened vermiculite under a piece of grate, a few holes around the sides - these are the same containers we use for hatchlings. Eggs go in the tubs, thermometer probe on the eggs, close tubs, close lid - don't peek for 60 days (well we actually look in pretty often!)
This season temps have stayed stable at between 82-84 degrees (they are in basement where room temp is about 75, humidity is always virtually saturated. Working well for us so far. Only issue has been the indoor/outdoor thermometer batteries failing (recorded higher temp in tubs than it really was) - used temp gun for weekly checks on temp.
Cost was about $40 for cooler, $60 for heater (Canadian $$), few $$ for ceiling grate and tubs.
mary v.