“...so that it is presentable to the eye.”
Thanks. I figured it must have been a limitation because of our eyes. So the pinhole in the cardboard trick is really just reducing the amount of total light I guess. But to see Venus one would need a bit of magnification as well.
Well, Venus is about 1 minute of arc, and the sun is about 30 times that, in diameter. So by usual telescopic standards, it's pretty easy to see as a disc. If we think of eyepiece projection, a solar image of 6 cm will show the silhouette of Venus as a 2 mm disc, which should be plainly discernible. If you're using a binocular, it doesn't take anything special to display this. The only thing I would rule out is plastic body toy or giveaway items. Anything substantial should fill the bill, even those small "spotting scope" type things, which are useful at the beach ;-)