Are you doing the dry hatch method? What I have found that works great is a meat injector. You just insert it into one of your vent holes and insert a small amount of water until you get the humidity to were you want it. Just make sure you don't get the water on the eggs.
They go inside the bator. On mine I drill a hole in the back and use a screwdriver so I don't have to open the bator. Make sure if you are using the water heater thermostat you do the modification with the extra holes drilled into it. I have started using this thermostat and it works great...
I have always built my on bators. The most I have ever spent was $45.00. The hatch rate has always been between 80 and 100 percent. The main thing is do your home work before you decide which way you are going to go. Google is your friend when you are looking for information.
Good luck and...
Today was hatch day for 7 Barred Rock and 7 RIR. I also put into the incubator 48 Golden Sex Link.
One of the Barred Rock.
One or the RIR.
And the incubator full of the Golden Sex Link.
I build my incubators and I have tried water heaters and wafers and they all work just not the accuracy that I think they need to be. I have switched to this item and it works great. It takes a couple of weeks to get the unit but it’s worth the wait.
http://www.ebay.com/itm/170848694225
For anyone that has a homemade bator or is thinking about building one this is the way to go for a thermostat to control your heat. http://www.ebay.com/itm/170848694225