You can cut the number of stitches, though I'm not sure I would do everything in half. The finished backpack depth is only 4 1/2", I would keep that (33 rows on the base), but decrease the number of stitches by 1/3 or so. Then you will need to decrease the number of stitches on body some to compensate for the decreased stitches on the base. Then you just omit some of the rows between the cast on and the bind off (not sure the flower pattern will work out that well on decreased stitches, I would omit that and do a solid body). Just remember that you will lose at least 1/3 of the height when you felt it. I am more of a wing-it knitter, adapting patterns on the fly, so sorry I can't be of more help with the math of the stitches.