I really hate wrapping presents so I developed tetrahedron gift boxes. In an earlier Instructable I showed how to make a small one that was 4" on a side. It came with a single page pattern that you could print on thick stock paper. This Instructable with show how to construct one that is 10-3/4" on a side using a piece of 24: by 28" poster board. As with the smaller ones, it assembles without any tape, staples, or glue. You will need a large compass, straight edge, rule, and scissors.
Step 1: Mark Start Pont on Poster Board
Align the poster board so it
is 28: high. At the bottom left mark a point 5-1/2" from the corner.
Step 2: Draw First Arc
Set the compass for a radius of 10-3/4". Place the compass at the start point and draw an arc across the bottom left of the poster board.
Step 3: Draw Next Arc
Move the compass to where the arc hits the bottom and draw another arc.
Step 4: Draw Circle
Move the compass to where the arcs intersect and draw a most of a circle. (It is cut off at the bottom.)
Step 5: Draw the Final Two Intersects
Place the compass at the most left arc/circle intersect and mark another intersect with the circle up and to the right. To the same thing on the other side.
Step 6: Draw Lines
With a straightedge and ball point pen, use the intersection marks to draw all the following lines. Note that the lines go pass the circle at the top to form another triangle. Bear down on the pen when drawing these lines. The creases will make it easier to fold when completed.
Step 7: Draw Tab Lines
On the top triangle draw two tab lines approximately 4" away from its edges. The photo shows compass lines but the tab dimensions are not critical. Freehanding it.is close enough.
Step 8: Cut It Out
Cut on the lines to get the shape shown in the photo. Note that one line (1 o'clock to the center) has been cut.
Step 9: Fold, Assemble, and You're Done
Start at the one o'clock point fold till you have an upside down tetrahedron with two triangles of paper on each side. Then tuck the tabs between these triangles to close it up. You are done.