A lot of the material we already had around the house so it didn't cost us anything but my time (about an hour) to shape and design the box. We plan on "burying" the gift somewhere in his house (we live in a city, no yards!) and we'll give him a treasure map of the floor plan to find it.
I had fun creating it and didn't stress about perfect corners much (he'll be four!), so there are no real measurements here. I'd love to hear how you modified this idea to suit your needs.
You'll need materials. I gathered:
a large cutting mat (something to protect your surface)
a box cutter (sharp blades really help)
a cardboard box (we recycled a rectangular medium shipping box)
roll of 1/2" wide double sided tape
roll of 1" wide black duct tape
thin point sharpie marker
12" L metal square ruler
clear scotch tape
approx 3ft x 1ft of brown craft paper
wide felt black and brown markers
a brown colored pencil
gold metallic marker
a gold metallic gift bow
some shiny yellow/gold plastic baubles
yellow/gold gift paper
First find a box that your gift sits nicely in.
Step 1: Step 1 - Create your curved top
Just above the "gift line" insert your sharp box cutter blade into each corner of the wall and cut up to the top. Be careful to position yourself so if your blade slips you don't stab your arm or eye, this is a pirate party not a slasher film.
Bend the cut walls over the gift line, you'll see the flaps of the cover meet neatly across the gift and already form the treasure chest dome effect. Hooray!