So, for now, I've done a quick mock-up with some copy paper to try to show you. The most important part is the center mechanism. I've worked out for a smaller mechanism and a horizontal card, but let me show and tell the standard A2 size. This particular size of mechanism is the most sturdy.
The card I've shown here is 6"x6" finished and the center pop up panel is the only change. I made it 3-1/2" x 11" scored at 2-3/4", 5-1/2" and 8-1/4". This width is a little too big for the A2 as it'll show at the bottom (the height - 3-1/2" here - can vary quite a bit but the cardstock "length" is very dependent on your card size).
Hope this helps some. If you have questions, please feel free to contact me.
Image is from Bloobel Stamps, Champagne Man
Coloured with Copics