Yay! The cover on the example is watercolour paper, but I forget what the gsm is, alas.
The securely unsecured format is a lot like one I learned that uses multiple tabs (through signatures) that lock in the same way (insert/bookmark) from a concertina inserted through a slit in the cover spine; the outside spine tab/flaps of the concertina strip could be left free, or folded and glued down. (and I was sure I had an example somewhere ... >.>;;))