I'm actually surprised at how many folks *aren't* using Sonotubes et al for their car audio projects. They are widely used for Home Theatre sub projects - large ones for the enclosure (18", 20" 24") and the smaller units for the ports (6", 8" 10").
Chad:
As per what the other poster stated, you create a "plug" for the ends - two pieces of MDF; one larger than the diameter of the tube and the second that fits inside the tube. Glue the two together and there you have it. Instant enclosure in a couple of hours. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
Here's an on-line Sonotube Calculator I use periodically to determine tube sizes for enclosures:
http://www.quux.net/roo/diy/sonosub/sonocalc.html
Sonotube ports:
Sonotubes are great for ports - they are cheap and come in a variety of sizes. As a couple of folks have mentioned, some brands have the "extended" sizes - one tube fits inside the other. These provide excellent choices of sizes since a '6' tube measures 6", '6+' tube measures ~6.5", '8' tube measures 8", '8+' measures ~8.5", etc.
Collo has extensive information on his site regarding port flaring. Very interesting read. He has also created Sonosub design software which is very good:
http://www.subwoofer-builder.com/flare-testing.htm
Regarding trying to flair a Sonotube, I think you would have problems since you are only dealing with cardboard, and we all know what happens to cardboard when it gets wet.
Best,
Mark