A guy where i live used to modify PVC birdhouses to make them more log-like. He sprayed them with 3M adhesive, sprinkled them with sawdust, and painted them brown when they dried. He had a bluebird trail in the park district, the bluebirds did use them, but he did say he had to touch up the paint on them now and then.
He had pictures and instructions on how he made them, if i come across them at home i will scan them and post them. I couldnt find anything similar online.
Chris