When I made oatmeal carton (cardboard) cameras I found that, in spite of sealing the base, covering the lid with contact paper and painting the inside black I still had leaks. I solve it by giving each carton an aluminum foil 'bonnet'.
It's possible to determine the size of a pinhole by using a scanner. I copied the following from a thread on the f295 website (http://f295.tompersinger.com) It was posted by Dvoracek.
"Put in on the scanner, set the resolution to the highest it will go, select as little area around the pinhole as possible and scan. Once you have the scan, look at it at 100%, 1:1, or Actual Pixels., enlarging it to 200 per cent doesn't hurt.
I you're using photoshop or photoshops elements, crop it down to just the hole itself, using select, cut, paste and then look at Image Size under the Image menu and it will tell you what size it is."
Once you get the size of your pinhole the calculators linked to the f295 site are quite useful.