Anet A8 fillament spool on top holder

I wanted to put the spool on top of the Anet A8 3d printer, but couldn't find one up to my requirements and decided to make a new one. The requirements are it to be:
- stable
- easy to mount, not requiring anything to be disassembled
- somehow flexible (to accommodate some differences in rod lengths)
- potentially upgradeable
- to provide mounting holes (pockets) for additions
- easily printable
- to be immediately usable after it is printed.

There were some holders, which were close t