To carry a BV500/Bearikade Blazer inside horizontally takes 42″ pack circumference. 44″ is better for long term durability, as it allows more padding.
A pack of that girth with a back height of say 24″ would be in the neighborhood of 60 liters. This is big for UL’ers with an otherwise compact kit, and the pack would be heavier than most can accept. Not to mention the important ‘tiny pack look’ of the times would be ruined.
One could devise an effective compression system like the Flex Capacitor to make it useful without a canister, but this adds more weight. I suspect most would have a second true UL pack for those times.
Or the pack could taper from the needed 44″ at the top to a more manageable size below, but the shape would be awkward and that leaves the full canister sitting high dooming the design with the top heavy feel most active hikers are best without.
Carrying the canister externally in a secure fashion is the natural progression to keep weight low, size small and have ergonomics trending towards comfort.
Makeshift strapping of a full or empty can on top of a UL pack not really designed for it, is the accepted thru hiker sort-of solution. Doing 30 mile days and comparatively not that much mandatory canister terrain on the PCT/CDT/AT this is totally workable.
But backpackers using a canister all the time needs something better.
Fong mentioned the Bears Ears pack which is based around, in the smallest frameless version, a 25 liter sack on top of a super secure harness able to carry any size bear canister. It is 21 ounces including a full wrap ‘floating’ hipbelt, and super easy water access. Sorry for promoting my own stuff, but this is the best concept currently if one wants to keep internal volume down and have a reasonably light pack. Replacing the canister with a food sack, whether Ursack or plain cloth, is an option to keep using the pack in less restrictive regions.