It will fit in an Eberlestock Big Trick, which has a scarab extension that can extended the pack length (zips away nice a tidy when you don't need the extra length).
@ohiochuck I’ve been eyeing one of these for some time now. I haven’t had a pressing need for an additional backpack to slip my Lelya 2.0 into. It fits into my day pack. Not sure how long your R3M is but it may work if length isn’t an issue. This company makes nice bags and cases that are affordable.
As @mtnGhost and @2muchair4leyla mentioned, Eberlestock makes some serious combat quality bags and packs. However, their packs are pricey as are their MOLLE accessories/add-ons.