You should end up with something like this:
Step 1: Collect the Materials
Lots of cardboard
2xPVC pipes (Rolled up paper will also suffice)
Some time on your hands
Step 2: Assemble the Stock
Once that is down stick the second piece of cardboard on top so it looks like the second pic.
And that's you stock done!
Note: A lot of glue is needed for this.
Step 3: Build the Shaft
Next, mark a rectangular hole for the shell, (optional) and then get some paper and roll it up so it can fit on the PVC pipe.
You're current inventory should be like the second pic.
After taping the shell inserting bit onto the stock, tape the whole thing together like the last image.
Step 4: Final Touches
Also, We added extra selotape for firmness and extra cardboard for the stock.
Anyway, customize this to your hearts content and leave us your tips, pics and advice in the comments!