Traveler's sacks are quality-dependent for their inventory boost, so why not backpacks as well? My thinking is that every 10 points above q10 should add another column of backpack capacity, providing extra holding capacity from better backpacks and giving newbies a reason to develop their leathermaking early on.
For example:
q10 -- 2 columns
q20 -- 3 columns
q30 -- 4 columns
etc...
Or perhaps the scale could start at one column, and each additional multiple of 10 quality would add a row, requiring backpacks to be at least q20 to have their current capacity.