Monthlong rhymes with:
budlong, chamlong, daylong, furlong, headlong, hourlong, weeklong, birdsong, budlong, chamlong, daylong, furlong, headlong, headstrong, hissong, hourlong, hussong, hysong, plainsong, scuppernong, ssangyong, weeklong
Monthlong sounds like:
medallions, meddling, middlemiss, middling, modeling, montelongo, mottling, muddling