Not a Retro or M-series, but my hollow 6 is rather thicker than most basses. It generally gets carried in the (original) case, but I'd definitely trust it to the Mono Vertigo gig bag I have, which is plenty deep and also very protective.
The only caveat is that it's really *too* big for my Petite 5, so it may depend on exactly how wide the body on yours is.
Thanks for the suggestion @mikezimmerman
My bass is a bit larger, more traditional than the Petite, so I should be good
Anybody here want to give it a go?