What makes a good cardboard box for this project:
Rigid / Relatively New
Not too big (12"x12"x12" of concrete weighs about 135+ pounds, think about the pressure)
Less than Ideal:
Find two boxes that fit inside each other with a 1" gap on all sides. More than 1" and the box could be quite heavy, less than 1" and it won't be as strong.
On the larger box (the main form):1.
Fold the flaps outward and tape them down.2.
Tape the inside edges of the box, especially the corners
Tape the entire inside of the box, with some overlap, in any pattern you want.4.
Tape cardboard edging on the outside wherever possible to help strengthen the box.5.
Add any decoration, like leaves, rubber stamps, pieces of tile, with spray adhesive.
On the smaller box (the knockout):1.
Fold the flaps inward.2
. Tape the outside of the box completely.
Place the smaller box inside the larger box. Lift up the smaller box until there is a 1" gap at the bottom. Make a mark on the small box, even with the rim of the larger box. When casting, you'll use this line as a reference so you know how deep to push the smaller box down.