So, total cost at this point (not counting trial and error):
Misc. Supplies: $10
So roughly $35 is what I could make this bag for. Add $15 for the backpack straps if they work out. I also found this online:
It appears to be the same bag, with some design changes, possibly better zippers, and an added small pocket in the front (mini pocket?). For only a marginal difference, I may pick this one up when I need to swap out just to see if it will be a more durable and workable option.