I've spent about a month working this out and hunting on the web but I'm somewhat stumped. What I would like to obtain is something similar to
this telescoping tube.
What I need is for the inner tube to have an OD no more than .43" (approximately 11mm) and the outer tube to have an OD to be no more than .60" (no more than 16mm) and obviously an ID to mate to the OD of the other tube. It would be really nice if the material is soft enough that I can cut them to length (plastic, copper, or something similar) but not so soft that it bends out of shape during use.
Shape isn't necessarily an issue. It could be round, square, rectangular whatever. Other shapes, I'll have to make measurement adjustments of course.

I looked at telescoping antennae (too thin and fragile), PVC pipe (walls too thick and requires extensive reworking, copper pipe (found 1/2" pipe which works out to be just shy of 16mm, but still trying to figure out where I can get pipe to fit inside). I have found mention of 1/8" to 3/8" pipe available in copper, iron and PVC pipe, but I'll be damned if I can find anyone that sells pipe that small. Looked at "rail" type fittings but they're unsuitable since I need to cut my pipe to length.
Any ideas on a source for this kind of thing?