Those fluorescent tube guards that Craig linked are quite light. I’d use them for some added stiffness and to prevent snagging for a pole set in a side pocket or within the main pack bag.
Slightly heavier, but still much lighter than, say, ABS pipe, would be the corrugated plastic tubes inside of a large rolls of paper (like the printer-plotter at an architect’s or engineer’s office). They’re kind of like the plastic advertising/campaign signs in being plastic (polystyrene?) inside and out with ribs between the inside and out that make them remarkably stiff for their weight. 2 to 2.5 inches in diameter, a tenkara rod set would fit inside easily.