Anyway, I found plans for the "sparrow spooker" online at www.sialis.org/sparrowspooker.htm and decided to try it. Everything I read was very positive, but I was still skeptical. I put it together according the "design #4" (basically because I had everything I needed just laying around the house), checked the nest yesterday (2 eggs), placed it according to the info online and waited to see what happened.
I hadn't seen either bluebird go anywhere near the box after I installed the spooker (although I wasn't watching it constantly), but when I checked this morning, mom had laid a 3rd egg !! She slipped inside while I wasn't looking, obviously.