Or, we use Latin names of numbers. So, Unusber, Duober, Tresber, Quattorber, Quinber, Sexber [rename as necessary for America], September, October, November, December, Undecimber, Duodecimber, Tredecimber.
Alternatively, you would rely on the ordinals, dropping the -us ending and replacing it with -ilis as in Sextilis and Quintilis.
Unusber