....here it is: the 16x20 pinhole. Well, kinda. I measured wrong and it's actually 16x19, so I'll need to trim the paper a bit. Gonna order some Ilford Direct Positive Fiber Glossy soon and give her a shot.
F/405 and effective ISO 3. Roughly 4 minutes in broad daylight. On the backside picture you can see the slot to insert the paper. I am going to insert paper, cover with electrical tape, shoot, bring back to the darkroom for development. Surely nothing can go wrong!
The angle of view is 77 degrees, which is identical to the 43mm on my 6x7, so I can use the external viewfinder for composing.
Let the fun begin!