I got some universal ones
Maypole 1.2m 90kg Locking Aluminium Car Roof Bars Cross for Profile Flush Rail
They are £55 and do the job , but I think any universal ones of the same style should work.
I used them and travelled a return trip to wales from Birmingham with a full roof box with no trouble and surprisingly quiet for roof bars..
This particular type has a quirk ( in my eyes) when setting them up ( this may be as they are universal so need a setting up time as all cars are different) , and I'm not sure if all of this type are the same.
It may sound strange how you set them up , but really it was easy and they have been great , but I think its worth letting you know.
Like some roof bars they have a rubber strip ( RSJ shaped) that slides into a groove on the top as a cushion which goes along the whole length on the roof bars from end cap to end cap , and this is cut to fit snuggly so no movement can take place, when the end caps are inserted at the end of the setup process.
Underneath it has the same configuration, but this will be cut into 3 sections and this is where you have to carefully measure the roof bars as the locking mechanisms need to be inserted in the underneath groove.
I did it by placing them on the cars rails so the locking mechanism sit on the rails and moving the bars as they are not fixed and move easily, so the overhang of the bars each side of the locking mechanism is the same. Once in the position I wanted I used a pencil to mark either side of the locking mechanisms on the underneath of the bar( this is very hard to try to explain something quiet easy lol ) . so you should have 4 pencil marks, 2 inner and 2 outer ones
So you then cut a rubber strip to fit snug between the 2 inner pencil marks ( this stops the locking mechanisms from moving closer to each other when they are in place ) then you cut 2 more between the outer pencil marks and the end of the bar ( to stop the mechanisms moving out).
So put the inner strip in , then the locking mechanisms , the the 2 out strips , then once the end caps are put in place , these hold everything together and nothing can move.
Again this does not sound very secure , but nothing has moved since I've done this.
This has to be done only once when you first have them , as you now have them set up correctly they will always fit.
As a note it took a good 30 mins to get them adjusted to the correct width of our roof rails.
Also note the rails distance are wider at the front of the car than the back of the car , so you need to do the set up on both bars ( not assume by doing the one it will be the same measurements for the other) and so you will have to place them in approximately the same place every time.
I hope this makes sense ( it does to me , nut i have done it lol )