If you want to make some and they do work get some 1 x 12 lumber. You will need to make a box at least 20 in. deep. Cut a 4 in hole about 4 in down from the top. Make the top removable so that you can clean them out in the next winter or after they have raised a brood. You will need to put about 3 or 4 in of sawdust or well rotted wood in it for nesting material. Mount them in a tree 10 or 15 feet off the ground and watch them in the spring if you can. The biggest problem is with starlings building a nest over the woody eggs and spoiling the woodies nest. Once you have a pair nest they will keep coming back year after year andthe young females migrate back to the same area where they were hatched so you will see more in a few years.