I've made a few tubes in micromax minimum diameter. I used legal sized (14" long) 20# computer paper for mine. For a mandrel, I took a 1/4" dowel and carefully wrapped a layer of Scotch Magic Tape around it (be careful not to overlap it). The tape gives a smooth, somewhat nonstick surface to work with and also increases the diameter enough that the motor will slide in without binding. I cut about 1/2" strips of paper and tape the end to the mandrel with masking tape so that it wraps around the mandrel in a non-overlapping spiral (tape both ends). The second layer goes on the same way, but the spiral runs in the opposite direction. I then thin some white glue with wate and use a paint brush to apply it to the underside of the second layer and wrap it in place, keeping smooth as I go. It will be strong enough for micromax motors if you want to leave it at two layers, or you can add a third layer for strength if you like (I suggest making some of both - you can then use whichever seems appropriate for the build). To remove from the mandrel, cut free the tape (be careful not to damage the tape on the mandrel) AFTER the glue is dry (don't try removing it right after making it or it will scrunch up some and distort). Carefully work the tube off the mandrel by twisting it off. Clean up the ends (cut off the uneven parts at both ends) and you've got a nice tube for minimum diameter models and for motor tubes.
One other source for paper I have used is cash register tape. It is also about 20# and you can reel off as much as you need if you need longer lengths than 14". You can also vary the width of the strip (up to a certain point where it no long wraps cleanly around the mandrel) to make longer or shorter tubes. Oh, and going in the other direction (larger motors), these tubes also make pretty good launch lugs for 3/16" launch rods - they're a bit tight on 1/4" rods.
