Billy Bookcase filament hanger

The hangers uses the original metal shelf pegs.
You'll also need two 23mm rods for each shelf. They should be cut at 745mm long.

I bought broom rods and cut them since it was cheaper than buying 1M rods. One Broom was ~4-5eur and enough for 2 rods, while a 1M rod was ~4eur and had to be cut anyway.

The pegs are easily mounted if you slide them partially in, then attach the holder before pushing them all the way in. 

As of right now, each shelf needs 8 pegs and 16 is included with the bookcase. However, at least here in Sweden (the land of IKEA), we can order spare parts for free. They are also available to purchase on various sites. e.g Amazon. Part number 131372.