The solution? Of course a bookend, but I haven't any money for this kind of stuff so I decided to build my own.
I know that it is possible to make some more stabil versions out of metal or wood but the only thing I got at the moment were a lot of cd-cases.
Step 1: Gather materials
an slim cd-case doesn't matter if it's transparent or black or whatever. I got 100 clear cases for 9,99€.
Some strong glue, I prefer cyanoacrylate, this glue goes to be hard out if it comes in contact with the water in the air. I got 3 gramm tubes for 0,25€ at a local store.
Something to hold the case in a determined position, I used an 3rd hand that you normally use for soldering. (look in step 3 for the matter).
A geometric triangle
Step 2: Bend over
Step 3: Fix it
Step 4: Glueing
Step 5: Let it dry
I just let it dry about night.