Mustn't rhymes with:
absent, adjacent, adolescent, antidepressant, bessent, complacent, complaisant, convalescent, conversant, convulsant, crescent, decent, depressant, devincent, dispersant, docent, effervescent, evanescent, fluorescent, hersant, incandescent, incessant, indecent, innocent, iridescent, luminescent, magnificent, melicent, melisent, mellicent, milissent, millisent, nascent, obsolescent, patuxent, poussant, quiescent, recent, reminiscent, reticent, stuyvesant, suppressant, translucent, vansant, vincent, vinsant
Mustn't sounds like:
mightn't, misstatement, moistened