I've seen, touched, carried surplus 60 mm plastic tubes. US has (had?) the 4 tube one, Canada-maybe has a 3 tube, maybe others but I didn't pay too much attention. 2-3 rounds (plastic or foam blocks keep rounds apart) per tube with a screw top, all glued/clipped together. Not unlike the two tube thing for the Carl G. Good ones have plenty of slots to strap to the back of a pack.
We've done a few FOF / game / training things where several of these were weighted as notional ammo and the team has to bring them to the "mortars," so get carried for hours or days, then dismount. With minimal planning and spare buckles/webbing, it works okay. Due to weight, much more comfy inside the pack, but then it's much harder to get them out. But since everything is heavy, take your pick.
I've always though a single tube (2-3 in a stack)* would make sense and hope whoever is working on this new lightweight system realizes it's a system and needs things like clever carrying tubes. To me: tube comes with many webbing tie downs, arrange it along the side of the pack, inside, across or whatever you want then. Bonus: clip several together, have integral handles and carry 6 or 9 or 12 several rounds like as briefcase for short hauls, bucket brigade movements, toss behind seats in a truck, etc.
I have many more questions and ideas (why not rifle grenades? why not spigot mortars? why 60 to confuse everyone? why not use modern digital tech to give a super-compact sight? ...), but totally past the end of my actual knowledge on any of this so will try to shut up now.
* Apparently this is a terrible idea. Don't listen to me.