What a cool project. It looks to me that it is similar to the
Tunnel Card tutorial we have.
Except for instead of a card, you adhere several 8 1/2" x 5 1/2" (scored at 4 1/4") pieces of cardstock together mountain/valley style. Then you cut windows in the center of 8 1/2" x 4 1/2" pieces of patterned paper and adhere them opposite of your cardstock (where your cardstock has a mountain fold, your patterned paper will be adhered on the other side with a valley fold). Does that sound like it could be it?