Good grieficus! That's a complicated circuit for something to beep a few seconds when the door is opened.
I've got a jar full of surplus Sonalert alarms. I'll be happy to send you one if they don't have any handy at your local Radio Shack. They sound when anything from 1.5VDC up to...well, higher than 12VDC, which is as high a battery as you want to use. You'll probably go with a 9V.
Start with a magnetic reed switch, NC. Now that's NC as electronic folks understand it: If you buy it from an alarm dealer, be advised that alarm industry nomenclature for NO and NC is opposite from the rest of the electronics industry. So the switch I'm talking about is NC = closed when not influenced by a magnet, and open in a sufficient magnetic field. If you buy from an alarm dealer, ask for an "Open loop switch", which they call a Normally Open switch but is actually a normally closed switch, as we understand the term.
Mount the switch on the door however you plan to do it, with its magnet in place to keep it Open while the door is closed. Again, this is opposite of the usual alarm switches, which are closed in a magnetic field. So the ones that go with your home alarm system probably won't work.
You need a resistor and a capacitor, I'm guessing about 1000-2000mF and maybe 10-20K ohm--not sure, but once you see how this works, you can do the math or just trial & error, whichever suits you.
You run the circuit Battery-->NC mag switch-->capacitor-->Sonalert---Back to battery. Parallel the resistor across the capacitor.
When the door is opened, the switch closes and the capacitor passes current to the Sonalert for several seconds until it's charged and blocks any more current. The resistor is large enough that it won't pass enough current to drive the Sonalert, but will drain the capacitor in maybe half a minute after the door is closed.
The fact that the sounder won't work instantly when you close the door is an upside rather than a downside:You don't need the sounder beeping every single time when the door is busy. It tends to drive people nuts. You need it to sound when the door hasn't been opened for awhile. You can adjust the resistor value to your judgment.
Sorry I'm not posting a nice schematic with this, but frankly the circuit is so simple I doubt you need one.