<human>: Objective: How can I make an extended magazine for a cosplay pistol?

Which of the following solutions is more sound in terms of naive physics reasoning?

OPTIONS:
- Make a box out of plasticard and shape it to the gun with sandpaper.
- Make a box out of Mastercards and shape it to the gun with sandpaper.
<bot>: Make a box out of plasticard and shape it to the gun with sandpaper.