What is the best way to add handles to a cardboard moving box?

I just purchased a bunch of moving boxes and want to add handles to them. I was going to use my razor and simply cut out two handles (one on each side of the box). I am just afraid that the box may tear when lifted (especially the heavier ones). What would be the best way to reinforce the handles. I am thinking I should place filament tape in the areas I will be cutting and cut through it.