Single 50mm UM2 spool holder

The original spool holder delivered with my Ultimaker 2 has room for 2 spools, which makes the total depth of my Ultimaker a bit too deep to fit on top of my desk. I will never put on two spools at once anyway.

So i remixed Redstacks 77mm design to a 55mm long design, so it will fit exactly one 50mm diameter spool.