it's simple, it's cheap and it's by recycled things
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Step 1: Materials
2x A4 cardboard
(if you don't have the length of the A4, than you take a blanc A4 paper and hold it on the card bord)
or : A5, A3, A2.... etc.
1x little rope
Step 2: Assembling
paste the 2 cardboard together with tape and make 2 holes (one on each board)
then pull the rope through the 2 holes as on the picture