You are looking for perfect here where you probably should be looking for "good enough". You will never build a chamber insulated well enough to hold temperature for hours or even perhaps a 24hours+ without cooling. Plus, you are never going to build a perfect vacuum chamber to prevent fresh air leakage or venting air the hot air into the room and that will also add to the cooling effect. If you have ANY opening or seam, there will be leakage.
Now consider just how big and bulky such an enclosure would need to be with the materials that a commonly available to you that you can afford. Is it really worth it for the small gain in finished product? Even manufacturers of high end printers accept the they will only get 'good enough' and that they need to have the heater cycle on and off to keep a steady temperature.