Comline 3161 heater?
 

Are there any heaters that are confirmed to fit in the first chamber?

The only heaters which will work are the very small preset heaters in the 10-50W range. These tend to be sold with a fixed temp of 77 and are not adjustable.

Something like this should work then right?
https://www.amazon.com/dp/B008L286BY..._t5_B008AGHI38

Its just a 10gal tank and I live in Arizona, so in the summer my heaters dont even run at all, and winter I keep the house at about 68F, so it would only need to heat <10F.

I would consider something like this as I am worried the flattened dimensions may make fit more difficult-

https://www.amazon.com/dp/B000OQO69Q..._t1_B00Q78MMT6