If you want a real stable temperature u need to have more isolation.
Make a hot airflow like a oven inside the inner chamber (heated one). Make that chamber isolated with some high temp material or use basic rockwool.
Next let cool air flow on the outer chamber to leave the outside of your printer cool so you can touch it.
We build a machine with 1m x 1m buildvolume and 200 degree chamber this way. Little bit more complex but for 90 degrees it's almost the same. If you need more help please contact me.