Canadian looking to move to U.S for unpaid internship

Hello-

I am a Canadian citizen, and I am looking to apply for an unpaid fellowship in the U.S.

The organization does not have the resources to process a student visa, however they are willing to write me an official letter confirming the duration of the fellowship, and its details for immigration purposes.

I would like to be fully prepared for this opportunity, so I was wondering if someone could advise me on the visa options I have available to me in the event that I am accepted to attend the 9/10mth fellowship?

I was informed by a U.S customs guard that all I require is an official letter from the organization confirming that the fellowship is for XXX amount of time, and I will not be paid. However, I want to be fully prepared if/when I cross the border.

Any information and advice would be greatly appreciated!