I found a deep box today (180mm) and it is a bit larger than 7x9, which is perfect.
I have being using coca-cola can aluminium to make the pinhole, but in this case I made the pinhole on the box cover and it may be thicker. I will make a bigger hole and place a pinhole made with cola can aluminium to see if it works.
I really like Jimmy's idea to put 5 pinholes. I may try it later on.