Have you had a chance to look at the modified Conley III martin entrance by Dave Yoder, Topeka, Indiana?
The entrance measures 2-5/16″ wide and 1.200″ in the center. “Along with the Conley III ,the third hole that Willie Conley designed, the two are being tested,” according to Andrew Troyer. The Conley II seems to be working well with only a few starlings getting through. The martins are still able to get through with little difficulty so it seems.
These two new SREHs will probably come it in quantity when the testing is done.
The above chart which I have completed shows the two new styles. Someday there will be an entrance which will not be breached by starlings.