The Airporter will work. We've used OD canvas duffles, huge sport-type duffles, the Airporter and another babkpack maker's pack duffle. I got an Airporter off Ebay for $12 with shipping.

I had a CampTrails external for a few trips that went into a surplus OD canvas duffle. My son still uses his. Just don't put the packed backpack into the duffle. Put the backpack in and then stuff in all the other gear around it.

(Yes, Philmont 5 times - what a trip!)