So you're trying to get a round peg in a square hole? or the other way?
If it's a round peg then you should be able to fill the square peg with putty and drill a hole the same size and your peg and you're done.
If it's the other way then I'd go and try to find some tube or rod the same size as the hole ( try hobby shops or hardware stores ), then cut off the existing square peg, drill into the piece a little bit to anchor the new peg and glue it in place with some plastic cement.
Failing that, run down to your hobby store or a hardware store ( or online if you're the patient type ) and get some rare earth magnets. embed one on each piece, being
VERY careful to make sure you've got them facing the right way and super glue them in. leave them for an hour or so to make sure the glue has set and you're done, pop the backpack on and off to your hearts content.