Here is how I made my filament joiner.
Simply use a soldering station, adapt a piece of aluminium with a 2mm groove on top of it.
Set the soldering station to the right temperature, use a PFTE pipe as a guide, and join the filaments !

If it fails, start again ! But it works most of the time.
Description here : https://www.thingiverse.com/thing:510...

I use it to join "pullstruded" filaments made with PET plastic bottles :
   • "PullStruder": how to make 3D printer...  
https://www.thingiverse.com/thing:496...