Due to a past job I have accumulated a lot (and I mean A LOT) of those tubes at the centre of receipt rolls. Most of them are plastic, some are very thick cardboard.
Several lengths, widths, lengths.
I also have some larger core from industrial packing tape that'd make the perfect base for towers or observatories.
I saved them because at the time I was really into terrain building and these just have so much potential it'd be a shame to toss them out.
I also have probably the equivalent of my weight in stamps so I'll gladly put them all in a box and send them anywhere in the world, for free. I'll even pick some cool stamps and make sure they're hand cancelled.
(I have already prepared the box and have foolishly forgotten to take pictures but we're talking about these things here, just many, many more than what's in this picture)