My first choice would be a Crown/Speed graphic 2x3. You can have multiple sized pinholes for different focal lengths, different backs for 6x6 and 6x9 roll film, separate viewfinder, find a bad lens off a MF or LF camera and mount it on the lensboard for your "bulb' setting and you can still use sheet film holders for whatever film or paper you wish to use. Usually under fifty bucks on EBay if you are patient.
My second choice is my crappy Ilford 'Sportsman' 6x6 with a single speed plus bulb shutter and the lens removed. It takes great pinhole shots.
tim in san jose