it ought to be constructed much like a bathtub floor.
take a rectangle of fabric that is the size of your base, plus the size of those side walls (4"?) plus seam allowance.
draw a 4" (or whatever side height you chose, plus seam allowance) square in each corner. fold each corner right side to right side, bisecting the 90deg angle of the corner, and sew along the two sides of the square which will now be lined up.
trim off the triangle of extra fabric, leaving seam allowance. fold over twice, sew through.
repeat for all four corners.
then sew this onto the already assembled tube that forms the upper portion of the pack. use the seam of your choice, i would choose flat-felled. make sure the seams on the bathtub bottom end up on the inside of your pack
think carefully about lower shoulder strap webbing attachment. ditto with hipbelt. both could be sewn into vertical seams of bathtub, if dimensions work.