This Birds Australia link shows how to build nest boxes with entry hole sizes designed for particular bird species smaller than a starling or mynah. Also includes a design for the anti-Myha baffle which would discourage starlings too. I have built one which worked to discourage mynahs from a nest box for the Eastern Rosella. https://www.google.com.au/search?q=Birds+Australia+nest+boxes&rlz=1C1CHZL_enAU730AU730&oq=Birds+Australia+nest+boxes&aqs=chrome..69i57.13102j0j8&sourceid=chrome&ie=UTF-8