I've been following this thread with interest, as I've been running into similar problems.
I've been thinking about doing something similar but using parts found in North America (save me a bit of cash on shipping & duty). I found a tube at McMasters that looks interesting:
http://www.mcmaster.com/#pfa-tubing/=hrc6gt
One of them is the 5/32" ID and 1/4" OD. Which is ~3.9mm inside and ~6.4mm outside for $4.33 per foot.
The other is in the metric side, 4mm ID and 6mm OD. Cheaper too at $3.15 per foot.
Are either of these fine to use, do you think?