This kind of overlaps the idea for “specific pockets” for clothing. You could craft these pockets, they would decrease the volume & weight of that one object in that specific pocket - but decrease the overall volume of the clothing suitable for everything else - giving you some net profit for having such pockets but you would “commit” to lugging that one item around to have the benefit.
Like this :
Originally:
Trenchcoat : volume 24
Double barrel shotgun: volume 12
Total : 24 volume, 12 available
After specific pocket:
Trenchcoat (modified): Volume 18
Pocket for double barrel shotgun: Volume 6
Double barrel shotgun (in pocket) : Volume 0 (already in pocket volume).
Total : 30 volume (18 free + 12 real volume of shotty), 18 available
BUT if you dont have shotty with you the trenchcoat (modified) would have only 18 volume, not 24 like normal trenchcoat because the specific pocket is always reserved away.