A simple loop prussik that works on 6mm ropes with a 3-wrap prussik hitch. The 5mm hollow-braid polyster cord is stronger than 6mm perlon, and it grips amazingly. It held my weight on Edelrid Rap Line, Mammut Glacier Cord, and 6mm accessory cord. When pulled until slip on the Mammut Glacier Line it always held more than 2kn. It also works very well on larger ropes. End-to-end strength is 12kn. 
 
The one pictured is a 28cm loop and it weighs 15 grams. Two of these are lighter than a Tibloc and much more versatile, and a lot less likely to damage your rope. On 6mm ropes it works fine with a autoblock hitch to back up a rappel. 
 
As far as what size loop to get? The minimum for use on just 6mm ropes is 20cm. The 28 will work on ropes up to 9mm. You may want it the same as some loop you already have or for something else, that's why it's your choice.
 
I can only get this cord in grey.