How to Make a Valentine's Day Envelope Mailbox

The back of the envelope will be cut out on the top lid of your box, so remove the lid, and set aside until step 5.

Lay the flat edge of the speed square against the side of the box, so that the 45-degree angle points diagonally from the top corner to the opposite bottom corner, and draw a light line with a pencil. Flip the speed square over, lay it against the other side so that the 45-degree angle points from the top corner diagonally down toward the opposite bottom corner, and draw another light line with a pencil so that the two lines intersect. 

Using the speed square as a guide, extend the blade on your utility knife and drag it along the straight edge, applying moderate pressure to the box top; stop where the two lines intersect. Repeat this step until the utility knife breaks through to the other side. Erase any excess lines that show. (Note: Don't try to cut through the box on the first try, making multiple passes over the same line will give you a nice clean edge without denting the box.)

Since the underside of the envelope flap will be seen, it's best to line it with a craft paper that goes along with the theme of your box. To cut the paper to size, flip the box lid upside down and extend the envelope flap so that it is open all the way. Slide the craft paper underneath and match up the corner of the paper to the tip of the envelope flap. Using a pencil, trace around the sides of the box, then cut to size. Feed the paper up through the flap opening so that it rests on the flap, trim and make any adjustments. Once you see that you have a good fit, remove the paper and spray a small amount of glue towards the tip of the flap. Carefully insert the paper back up through the opening and press it into place. (Note: It's important not to spray the bottom of the paper because you'll need to allow the paper to adjust to the bottom of the box once it's back on.)

Place the lid back on the box, then press the paper into place, creasing it along the inside bottom corner. Creasing the paper should help it stay in place, but it's also a good idea to reach down inside the bottom of the box and add a few strips of tape for good measure.

Cover the raw edge of the envelope flap with washi tape of a wide width.

If the box is sitting on top of a desk you can customize the front, otherwise send a pack of picture hanging strips with your child so they can hang the box from their desk knowing it won't fall off no matter how many goodies are stuffed inside.