Ok ... here's a quick scan of the contact print.
postcard is larger (longer) than the 4x5 neg and (if you click on the image above) you can (just) see base fog in the edges of the negative.
Hopefully the can will reduce the amount of light coming from the source lamp to allow me exposures of (say) 10 seconds. I'd like to target something like that figure to allow more accurate timing of my exposure.
Perhaps I'll need a lower wattage lamp too ...