Marvin has dedicated parking in Lovejoy Garage located at 1331 NW Lovejoy Street, Portland OR 97209. The garage can be most easily accessed by walking up the car ramp or heading up through Safeway. He is parked on the P4 or P5 parking level (the first level is for Safeway customers) in spots 401-556.
His keys stay inside, and hand-off can be done remotely to unlock at the start of your trip! It's cool and makes renting super easy as I can accommodate just about all pick-up or drop-off time that works for you.
If you're coming from Portland International Airport (PDX) Downtown Portland is also easy to access. Just hop on the red line MAX ($2.50) and get off at either the downtown China Town stop and walk, or the Galleria/SW 10th Ave Max Station and transfer to either the NS Line or the A-loop of the street car (same ticket). Then exit at NW 10th and Johnson street and walk the 5 blocks to the lot.