Just a "FWIW",
I've been using PVC pipe capped on both ends since UPS cracked the FF handguard on one of mine a few years ago.
I put an old shop rag, or wash cloth at the one end, then put the upper in - barrel first, & fit as much packing as it'll hold. I use "Bushmaster style" packing (My shredded bills!) for cushioning.
Then I cut the PVC 4" - 5" longer than it needs to be, put several rags against the back of the upper (until they're even with the top), cap that end, and mark it with a fat magic marker where the pipe & cap meet.
The reason I make it too long and mark it is so I can tell the buyer to "cut the pipe at the green ring". That way I don't have to worry about him accidentally cutting the upper. The reason I don't point the barrel at the end with 4" of rags, is because they (barrels) could sneak up through the rags.