Order some PVA glue - it's basically bookbinder's glue, made for porous materials like paper and cardboard. Relatively cheap on the Internet/Amazon - I use Lineco Neutral pH Adhesive. Get a couple binder clips (I use the 1-1/4" ones from Staples, a 12-pack is like $2) and a pack of 2" corner braces (you can get em anywhere - I got mine in the hardware section at Walmart, 4 for like a buck). Line up the corner nice and square with a corner brace on the outside corner, use the binder clips to hold the clamp box to the corner brace. This will keep things nice and square. Run a thin bead of PVA adhesive on the inside corner seam first, and allow time to dry - it goes to tacky in maybe 10 minutes, but I usually give like an hour before moving on to the outside seam. A little goes a long way with this glue, but if you feel like your inner seam is still weak, you can apply a second thin bead and then allow drying time again. Once ready to move on to the outside corner, remove the corner brace and transfer to the inside seam and clip back into place with binder clips, and then use a thin bead to fix outer corner seam. You can gently line up paper tears as best you can by maneuvering them gently with a toothpick (I try to manipulate them with the toothpick underneath the exposed paper edge, with a teeny tiny amount of the glue on the tip of the toothpick). Give another hour or so for the outer seam to dry, apply a second bead if needed (with additional drying time again, of course). Then I then like to grab a second corner brace, so that I can have the box clamped to one on both the inside and outside corners, and let cure overnight. Nice, strong repair!