recent mistake here (I did read the thread before and it didn’t stop me :P): I filled the static tubes with epoxy concrete for crush resistance (the only easily available 25mm tube around here is the kind you hang curtains on, chromed 0.5mm wall thickness steel tube; it is flexible enough to be deformed by the bearings, and tends to become a bit sloppy eventually); the filling did achieve the “crush resistant” goal, dampens vibration really well, but I got the concrete composition wrong (gravel too large, not enough fine sand, maybe not enough epoxy), so after a few trips of the gantry the exterior steel took the shape of the underlying pebbles / small voids and looks like the surface of the Moon 
I located some 25mm / 2mm wall steel tube on the website of a hardware store nearby[1], but I need to go there and see if it’s straight, and if they’re willing to cut the thing into 2m pieces (because using public transit with a 6m pipe is… awkward, and it wouldn’t fit the elevator anyway)
[1] it’s the first time I see such a thing actually, usually they carry inch sizes for water / gas / etc., and those use some funny math - 3/4 in is 26.7mm, no big box store sells stainless, and stainless suppliers won’t even talk to you unless you buy like half a ton (no, really, I added some bar to the cart at one of the rare ones who had an online shop, and the cart refused to let me pay saying “you have 2 kg in your cart, please add 498 kg more”); the joys of living in Eastern Europe, I guess.