Google cardboard recycling. Most cities have at least a few cardboard collection sites that have dumpsters you can raid. Amazon boxes are okay, Chewy boxes are fantastic. Get a few rolls of fiber reinforced kraft paper tape <link> and a sponge (this stuff is water activated). If you put one strip along the seam, and one down the center of each bottom flap, even a flimsy cardboard box is unlikely to fall apart. Total cost: $10, and maybe a minute of work to reassemble/reinforce each box.