For the corners you could trawl DIY / bits in bobs stores for a corner cover. I'm thinking like the sort of thing professional loudspeakers have, the kind of chrome/3 sided cube thing to protect the corners. That sort of thing would hind the joins of the piping. Probably look best in Matt black if you could find that sort of thing somewhere. Check out bath seals, they sometime have corner fixing that may work.
As for the piping, I bet your car seats already have examples of it stitched into them. Try and find a type that matches and it'll look matched to the rest of the car. The best I could see was the picture of the car with 2 speakers and the tape on the back of the seats above, if you look above the tape it looks like some black beading on that seat.