I don't like the pinhole idea because it limits the light that passes through, substantially. And 20 mW is barely enough for decent speed prints. I prefer the lens approach. This one on ebay has a focusable lens. At least this way you get the full 20 mW instead of about 3 mW through a pinhole.

http://www.ebay.co.uk/itm/405nm-20mw...item58ae5e9eed