Perhaps you will need to look for a good NEAR rhyme — esp. one where the vowels match exactly but something is added to or taken away from the middle consonant cluster /tr/, or another consonant is substituted for the /t/ that is not too far away from it in sound, e.g., /d/

ephedra

centra, dicentra

plectra, spectra

extra

fenestra, palestra, sequestra

tephra

Debra/Deborah

septa

Still difficult, isn’t it? Maybe “extra” (or spectra?) will work for you.

(By the way, the term some have listed as “et cetra” should be “et cetera”. With the extra syllable, it may not quite work.)

intra

kama sutra

Petra

Contra

I assumed that you wanted English words. Or was it Spanish for letter?