In the winter we keep the thermostat in the house turned down during the day. My home office rarely gets the heat turned on (we do live in a relatively mild climate here). What I do find is that if the door to the office is closed, the heat from my 27″ computer, 27″ second screen, router, modem, audio system, external hardrives, etc. actually warm the room up quite a bit – like 5-6 degrees. Now that I had my cat taken care of with the heating pad, there was the problem of the cat wanting to leave and come back to the office on a regular basis. And I didn’t want to have to open the door for her every time.
I have a chin-up bar installed in the door frame of my home office. What I did was take a short piece of cord (make sure it can support at least 5 pounds or so which I tied to a coat rack on the back of the door. The closer you can place this to the door hinges, the more it will be out of the way and the less it has to go up and down to close the door, but the more weight that will be required to do the work.
On the other end of the cord I just hung on old metal water bottle over the chin-up bar and adjusted the amount of water in the bottle so that it was enough to just close the door but not too much weight that it closes the door quickly, catching the cat or her tail in the door behind her.
The effort required to open the door is very little and our cat quickly figured out how to open the door. The counter weight only moves about 4″ to close the door. If I want to leave the door open, I made a hook from a clothes hanger that wraps around the chin-up bar. I just lift up the bottle, and hook the top of water bottle.
Not everyone will have a chin-up bar, but you could easily use a piece of 1/2 or 3/8″ wooden doweling inserted into the doorframe (either on one side, or a piece that spans the entire doorway) or a long screw/nail.