Tony
Steve makes superb cables -- and the dealer supplied coupler will also work.
However, there is no reason for you to buy these. You can easily, quickly and safely splice a length of cable in, with no loss of fidelity or functionality.
There is sheidling to be aware of - but you can keep the join to a few mm if you are neat - and this is no more "exposure" for the signal wire than is found in each plug/socket, or in the various cable junction boxes around.
Just make sure you connect like for like - there are various types of PL.
Ideally solder the joints - but careful "twist & tape" joints will also work ( and you can re-do the braided shield easily ) as long as you take care to avoid any shorts, and do not put the join under tension, risking it pulling apart.
I have installed HUNDREDS of PL cables over 20+ years, and have on occasion spliced cables for space/time reasons, to no detriment at all.
Others may tell you different - but this advice is usually followed closely by an invoice.