What is the correct way to pluralize a proper noun like "Raspberry Pi" (a type of small, inexpensive computer)?

Would you say:

> 
> * This project uses two Raspberry Pis
> * This project uses two Raspberry Pi's
> * This project uses two “Raspberry Pi”s
> 
> 
> 

None of the options look right to me. I think the combination of being a proper noun and an unusual second word makes this tricky.