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Recently, I designed a cardboard costume for someone who is in middle school. So my group, going along with the assigned theme of Marine themed costumes chose an individual theme of Finding Nemo. We all chose significant characters from the movie, and the character i chose was, my personal favorite, Crush! So I designed a sea turtle costume that a child could wear for say, a play production of Finding Nemo! But we did have some specifications and limitations for the costume such as:

Specifications:
Should...
-be made entirely out of cardboard
-use slotted construction
-be flat pack
-fit a middle school aged child
-use no adhesives

Limitations
Must...
-be safe
-be easy to assemble
-not require too much parental supervision
-use no adhesives
-be flat pack
 
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Step 1: Materials List

Supply list
ITEM    MATERIAL    SIZE       REMARKS - PART
S1        Cardboard     6x3         Shell, shoulder straps

Tool list
ITEM TOOL                                 USE
TI CUTTING MAT                       To cut the cardboard on top of
T2 UTILITY KNIFE                      To cut the cardboard
T3 RULER                                   To measure lines and cardboard
T4 PENCIL                                  To make marks and lines
T5 PROTRACTOR                     To measure angles

Step 2: Get Some Cardboard!

Part 1: Turtle Shell
Your going to need a sheet of cardboard that is at least 6' by 3'

Step 3: Measure the Oval!

Picture of Measure the Oval!
Part 1: Turtle Shell
Measure out a 4' line on the cardboard. Make a small mark on the halfway make (or at 2'). At that mark, make a 2' horizontal line that is centered on the vertical line.

Step 4: Make the Oval!

Picture of Make the Oval!
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Part 1: Turtle Shell
Now, were going to make the template for the oval. Your going to need a large piece of paper. On it, your going to measure up one side to 2' and mark it, and from the same corner, measure 1' out. Connect the two points with a curved line. Then cut out the template and place it in the angle where the lines on your cardboard meet. Then trace the curve. Repeat for all 4 parts of the oval. 

Step 5: Make the Inner Oval!

Picture of Make the Inner Oval!
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Part 1: Turtle Shell
By now, you should have your oval completely traced. Next your going to make the smaller inner oval. Your going to repeat the same steps from the previous step( making the template) only instead of 2' and 1', your going to make the sides 1 1/2' and 3/4'. Then, do the same as your did in the previous step, and trace the curve for the oval. 

Step 6: Make the Silts!

Picture of Make the Silts!
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Part 1: Turtle Shell
In this step, you're going to need a protractor. From the line(doesn't matter which one) measure and mark angles at 30 degrees and 60 degrees. From the center point, trace a line outward to the end of the oval, on the marks. Repeat for all for parts of the oval. You're also going to want to trace the 4' line so you remember to slit it. BUT DO NOT TRACE THE 2' LINE. All the red lines in the picture are the ones that will be cut. You should have a total of 10 slits.

Step 7: Cut the Oval!

Picture of Cut the Oval!
Part 1: Turtle Shell
At this point, you can cut out the large oval with scissors or a box cutter. 

Step 8: Cut the Slits!

Picture of Cut the Slits!
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Part 1: Turtle Shell
Now, with your scissors or box cutter, cut the lines you traced from the angle ONLY FROM THE INNER OVAL OUT. Do not cut inside the inner oval yet. So for now, just cut outward from the inner oval to the edge of the cardboard.  

Step 9: Cut the Small Slits!

Picture of Cut the Small Slits!
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Part 1: Turtle Shell
Next to each of the 10 slits, your going to measure 2" out and make a mark near the edge of the costume. Then draw a line going through that mark that is 2" long. So you should have a 2" parallel line, 2" away from every slit.  Than you should have a small flap from the slit you just made. F(flap A) Overlap the large pieces made by your 10 large slits. Mark where the fold is on the smaller slit and trace it onto the adjacent piece. you should have a diagonal line on one side of your large slit, (Slot A) and a parallel line of the other. You can now cut the diagonal line. Use the pictures to help you with this step.

Step 10: Cut the Arm Strap Slits!

Picture of Cut the Arm Strap Slits!
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Part 1: Turtle Shell
Measuring from the top an bottom of the inner oval, measure down 9" and make a mark,and then to 13" and make a mark. Repeat from the bottom. At these marks, make horizontal lines. on each of these lines make marks at  4" and 5" measuring from the CENTER LINE. (main vertical line)then connect these marks between the top 2 lines and the bottom 2 lines. See picture for guidance.    (picture has three slits but you really only need two) Use you knife to cut these lines, creating slits. 

Step 11: Make the Arm Straps!

Picture of Make the Arm Straps!
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Part 2: Arm Straps
Measure a line on a piece of cardboard that is 18" long. Make a mark on the halfway mark or at 9". At that mark, make a perpendicular line that is 18" long. Make a mark on the 15" point. On the first line you made, make marks at 0", 4", 14" and 18". From those marks, measure up 5" to make more perpendicular lines. From the outermost lines you just made (0" and 18") Connect those points to the end of the 18" inch vertical line, making a triangle shape. Now you're going to repeat this step only using the innermost 5" lines, and the 15" mark on the vertical line. So now you have 2 triangle shapes. Erase the lines that are erased in the picture. You are left with the strap shape(like a V). Cut this shape out using scissors or a box cutter.

Step 12: Assembly!

Picture of Assembly!
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Now we are going to assemble the turtle shell. First, (part 1: turtle shell) going back to the small flaps you made on the shell, you're going to insert those into the diagonal slits you made. Flap A into Slot A. Fold the extra of the flap back to lock it into place. Next, you're going to insert the arm straps (part 2:arm straps) . into the outermost arm slits you made. Then fold the remainder of the flap into the innermost slits locking it into place. Repeat for all arm strap flaps. After you have finished the arm straps, You're finished!