I have just done this on my mk4 FR and I used some windscreen washer tubing that I bought from halfords a while ago!
I used the 4.7mm internal diameter pipe and it fits nice and snug in the gap, I must admit that the difference in wind noise is much better than what I expected!
This is a must do mod for anyone! Plus you can buy the pipe for £1.50 for 2.4M at the moment (bought some more today as I only had enough left to do one side), the 2.4M is more than enough to do both sides!