Other possible rhymes for immature
catur
cature
cock-sure
cocksure
decatur
disarmature
disinsure
ensure
gridsure
insure
insuree
makedamnsure
mature
missure
mnsure
nonmature
nonpremature
overinsure
overmature
oversure
oxysure
pot-sure
preassure
premature
reassure
reinsure
sarab-e-sureh
semi-mature
semimature
sure
sureh
underinsure
unsure