@clk75201, I took the images you shared and made a template for the bag in Inkscape. Saved an SVG and a PDF in the Committees Drive (creativearts\Leatherworking\Cindy K Strap Bag folder).
You should be a good jumping off point to either:
a) print out a hardcopy, transfer to cardstock, and use as a template to manually cut out of leather or
b) cut the leather out on the laser.
Using the grid on the green mat as a guide (and assuming it was 1 inch per square), the finished piece came out just barely over 6"x13". Assembled, the compartment would be about 4.25" square and .75" deep (e.g. about the size of a man’s bifold wallet). More of a mini-bag than a bookbag.
You’d need a piece of leather at least 7"x14" and some 1/2" strapping to make the bag.
I also scaled up the pattern (assuming a 2" grid in the original). That one would need one inch strapping and would finish out with about 8.5" square by 1.5" deep interior. Both versions are on the shared drive. I think this one might have more practical applications and would work for a purse (or a cosplay/renfester/mountain man possibles bag). This one would need about 13"X27" of leather and 1" strapping.
NOTE: The red square on the pattern is approximate and might need to NOT BE USED if you plan to use a metal grommet in the bag for the strap that closes the bag.
Here’s a preview:
Cindy K Strap Bag one inch strap.pdf (37.8 KB)