This doorstop is the idea of Purple Martin Society member, Peter Menini of Island Lake, IL.  Yes, it is as simple as using a styrofoam cup in the holes to plug your wooden or metal martin houses closed.  Or you can make doorstops from a soda can.

Here is the information!

(For wooden or metal houses. Tabs would have to be made longer for wooden houses.)

Turn sida can upsidedown. Look down at the bottom of the can. All the shiny aluminum you see is wht we need. Cut side of can as shown to remove bottom  (fig.1)

Turn bottom of can up so you are looking at it, then cut aluminum as shown to round part A (fig.2).

With bottom of can still facing up, pull each tab B up. You now have your door stopper. Now, the joy of your labor. Put the doorstop in with the tabs going into the entrance from the outside (the tabs should be on the inside of nest area.) With one hand, hold the doorstop from the outside and with the other hand push tabs down. The tabs will lock the doorstop in position.

Please recycle the rest of the aluminum can.