An electromagnet would work, make the electromagnet hold the door shut when it is not energised, then have a button to energise the magnet when you want to open the door, the magnet is then only on for 1/2 a sec, to prevent unauthorised acces hack on of those plug in timers to switch dc, you can get them where the clock section runs from a button cell, its not so tricky to find where the o/p is to the switching part, or you could probably find something on ebay.