Connecting Tubes: How does one do it?

Suppose you have a long distance you need to fill between the front of a plunger tube and the back of a barrel. Like, you're making a Kenner 95 Crossbow to the original configuration with the barrel on top. Sure, that's a lot of dead space, but you're gonna try it anyway.

...how would you actually do that, anyway? What kind of tubing? What kind of material? How big?