Best M48 or M42 threaded Barlow / Focal extender ?

Hi again community,

I am looking for a good quality focal extender / Barlow - is there an optical difference ? - (that don’t produce much coma).

I want a fully threaded solution, as this is just for astrophotography - and just have a M48 thread at the rear of my quadruplet (TSOptics Imaging Star 71/347).

Looking for x2 to x4 focal extension (more x2 than x4, but open to any suggestions).

Any advice ? Any reference pictures ?