Depends on the scale of the model, how scale you want it to be and the era. As mentioned a rubber tube slit down one side works. Proctor offers several sizes of a shaped tube that is slit. You can find them here: Proctor Cockpit Trim

If you're an anal scale fanatic like some of us here, some thin leather works and is typical of the real thing. Finding it can be a challenge but I've had luck in the past obtaining scraps and leftovers from these people: Fox Creek They also make some fine motorcycle gear if you're in to that.