My Latin-Dutch dictionary (Muller-Renkema, 1970) lists the debauchery, without the macron, as first lemma, saying it’s related to lŭtum, mud.
The sacrifice, with the macron, is related to lūstro, which in itself has two meanings.
illuminate, derived from a reconstructed form leuk-s-trom
purify, derived from a reconstructed form lŏu(e)s-trom cf. ἐ-λοϜεσ-σα ...
Any syllable that ends in a consonant (one or more) is heavy
The terms "length by position" and "length by nature" are often avoided in more modern work. It's better to speak of "heavy syllables" and "light syllables". As described in the answers to a previous question, What makes a syllable "heavy" or "light"?, if you follow a certain ...
Don't just look at stresses, there are languages with long and short vowels. Czech, Slovak and Hungarian uses: banán, Latvian uses: banāns.
So to answer: I would copy that and make the first short and the second syllable long.
This is following up on Rafael's comment.
The Romans did not have bananas, but the botanical name musa occurs in Latin since at least the 13th century, originally as a transcription of Arabic mawza in the Latin translation of Avicenna’s “Canon”. Later, it was adopted by Linnaeus as the (still current) botanical name for the genus that includes bananas. So ...