Rat snake got into my bluebird box on Tuesday and I lost all 4 chicks

I check my 3 bluebird boxes every morning before work, and Tuesday around 6:45am I opened the one on the back fence and found a black rat snake curled up inside with a bulge in its middle. All 4 chicks were gone, and the snake had pushed past the baffle I put up last spring. I grabbed my trash can and a pair of gloves and got the snake out without hurting it, then drove it about 2 miles down the road to a wooded creek area. The baffle was mounted too low, only about 3 feet up the pole, and the snake used a nearby shrub to bridge the gap. I moved the shrub, raised the baffle to 5 feet, and greased the pole above it with a 4 inch band of petroleum jelly. If you run boxes, go out right now and check the height of your baffle and look for anything the snake can climb from. What height do you all keep your guards at, and has anyone had luck with the cone style versus the stovepipe?