We turned a teacher friend's classoom into a camera obscura by blacking out the windows (I think we borrowed some big black garbage bags from the custodian)and taping white paper to the opposite wall. Or perhaps we used a portable board on wheels to experiment with the lens/filmplane distance. It was a while ago.
We used 'lenses' of various sizes - just circular cutouts in black paper, I think the dime-sized aperature worked best. We had a nice inverted view of the traffic on the highway outside the room.
I've also used 4x5 inch sheet film holders sheet film holders
in a wooden camera built to fit . Polaroid film holders will give you instant results.
There is a product called Liquid Light Liquid Light that you can coat onto various surfaces to make them light-sensitive. I've known people who make pinhole cameras from empty eggs using this stuff.
As far as exposure times - I used the Mississippi method - keep the shutter open for 3 or 10 or 20 or 60 Mississippi's, depending on your apeature and film speed.