A clear tape. Costs what 20 cents, weights nothing.

I use 3M clear shield. You can buy various widths by the yard for relatively cheap. On my carbon frame, I applied it to the stays and used it as a chain protector (worked well).

I use Jagwire mini tube tops:

http://www.ebikestop.com/jagwire_mini_t ... CA2250.php" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Works really well but you have to put them on when you are installing the housing.

They also have larger ones that snap onto the housing but those seem to leave rub marks for me, the mini tube tops do not.