I've been working on this secret slider card. As I noted in a prior post, one of the design challenges of the secret slider mechanism is that the flap that covers the secret image will just cover the window if it were perfectly flat.
As the secret slider is used, though, it will not lie perfectly flat -- it will develop some pre-creases (which will actually help the mechanism work nice and smoothly), leaving a small gap at the bottom of the window.
To remedy this, I tried extending the bottom of the "plaque" that pops up to cover the gap.
It worked great!
I had concerns that the extension would "catch" on the window, but it works very, very smoothly.
This new birthday party invitation is nearly done -- I'm still poking around at the graphics that appear on the tab that you pull out as I'm not happy with that yet (and I may add in an arrow or two to give the user a clue as to what to do, though everyone that I've handed this to has been able to figure it out without additional visual clues).