well, coax would be the simplest way, since no harmonic filtering will be needed to return it to being a mostly sine wave.
You could still do things differentially, i.e. an ANALOG differential output amplifier, driving shielded twisted pairs. At the receiving end, an ANALOG differential input op amp.
But now you are starting to approach the cost of a coax cable. I mean, what does a coax cable with RCA connectors on it cost, maybe 50 cents in production quantities?
you say the boards are "in a tube". you could try something out of the box, like linearly modulating an LED with the sine wave, then using photodiodes on each board to receive the modulated light. It would not be super linear: i.e. would generate harmonics, but not as many as a true digital signal would. if the light will not propagate well inside the tube, you could use a plastic light bar....i.e. a piece of clear solid plastic tube inside of the bigger cylinder....distributing the light, a flexible light pipe.