Contractible spool for filament coils

The filament I usually buy always comes on a plastic spool. But recently, I bought filament from some other source, and that one came rolled up in a zip-tied coil. Even though I do have empty spools, I did not feel like unwinding the entire coil just to re-wind it onto an empty standard spool...

So I designed this simple contractible spool for loose filament coils, i.e. filament which comes rolled up without its own spool.

The idea is to have a spool which can easily be contracted for