I took a 23 ounce, 1100 cu in old book bag that I modified for use on the trail.
I cut off all the bells and whistles, used a seam ripper to remove the pockets, and cut out the bottom of the pack. Then I sewed a piece of Silnylon around the base of the pack.
I then sewed the sil closed with a straight seam, then boxed the corners just like I was making a stuff sack.
I ended up with a 1800 cu in panel loading pack that weighs in at 11.6 ounces for about an hours worth of work.
I'll add some pockets when I get a chance that should boost it to 2000 cu in.
Your only limited by the size of the original book bag ... if you started with, for example, a 1700 cu in pack you could certainly be pushing 2800 cu in with no problems. Just buy as simple a pack as possible.
And it saved this pack from the landfill