"Yes, hollow metal tube rings. Solid metal bar (round, square whatever) rings as well and it is higher mass.
Wood doesn't seem to ring and can be quite heavy depending on what species.
Going round the farm and tapping various things to see how they react (ring, frequency) I have it down to two or things which seem to be the best.
Wood - solid, rigid and doesn't seem to ring but can be heavy
Plastic water pipes - hollow, no ring at all, extremely light .
So, black plastic water pipes of about 20 - 30 mm diameter seem to be the best that I can come up with and are dirt cheap. Maybe they can be polished to look attractive or may require a finish adding. They are vanishingly light (super light), easier lighter than similar lengths of solid wood or solid metal. Solid metal still rings
For the lengths I need (40 cm / 16") they don't get any chance to bend. I can insert some very small inserts in the ends that can take a bolt for fixing at the top and bottom. Instead of a square 30cm top plate, I can use a + (plus) shape to link all the four uprights and keep it stable. That will keep it light and less material to ring (depending on what I use). The support at the top will have to be slightly different as a + (plus) shape will interfere with the down firing mid bass."
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I had come up with a similar idea but it entailed using metal inserts glued into the inside of the plastic pipes. I would drill them a put a thread inside the hole. They could then be bolted on from inside the enclosure. In fact using your idea the wooden inserts could be glued inside the pipe and avoid using screws.
