I picked up a Dewalt backpack at Home Depot designed a hold small power tools and many attachments which works perfectly for holding magazines, the sub 2000 plus I’ve got half a dozen 33 round sticks, a dozen 17 round and a 66 round drum
Correct I found that the construction backpack I picked up is double or triple the strength of even an L.L. Bean backpack and yes I carry a Glock 17 or 19 and all of the magazines for the sub 2000 including the drum fit both of those handgunsIt truly depends on how you travel and how often you will transport it. Do you normally wear a backpack? If so, use a backpack. The main advantage of one is that it keeps your hands free for the 99.999% time you won't be pulling that S2K out. I assume you carry a holstered handgun and that would be your first go-to weapon, if necessary, anyway.
If you transport it daily, get something that can withstand the wear and tear. They make some good quality backpacks these days if you shop around.