I stumbled across those large envelopes today at a paper store and thought they would be perfect for this project.
The envelopes are 16 x 22cm (which is about 6.5 x 9 inches) and I only used 3 of them instead of 5. I glued wrapping paper to the inside of each envelope and left the outside as it is. Next time I think I'll use colored envelopes and draw on them or rubber-stamp them with self made rubber-stamps.
To attach the string I used an eyelet and cardstock on the back to make it more stable.