Mostly because I used a giant cooler as an incubator. If you look up home made incubators, here and on the internet at large, you will find most are using a cooler as a insulated "case". This simplifies a lot of the building. For mine, I didn't even drill any holes, just passed the electrical stuff out of the drain plug, and intentionally don't make it too airtight so that there is enough oxygen exchange.
I do run the hatcher a little cooler, I aim for between 98 and 99, whereas the main incubator is between 99 and 100. Sometimes I find the temp dropping to 97 or even to 95, when I add cold water or leave the top open too long.