Nice, I love this DIY stuff! I just made a Mic-Flag using a $0.50 box from Tap Plastics, some black spray paint, and 4 stickers. Normally they run around $30+.. my cost was less than $5.
I like this design because it's simple, very compact, effective (similar to the EPIC camera housing), and not much to go wrong or break off.
Normally I consider Tap Plastics
to be pretty pricey, but you might want to check them out here and compare prices with your supplier. I'm sure with some shopping around you can get even better prices for materials. Extruded Acrylic Tubes
(it's not CAST, but does that make any difference here?) http://www.tapplastics.com/shop/product.php?pid=141&
4" OD x 6' w/ 1/8"
wall is $55.50
4" OD x 6' w/ 1/4"
wall is $96.55 Laser-Cut Clear Acrylic Circles http://www.tapplastics.com/shop/product.php?pid=140&
4" x 0.235" thick is $2.00
If you can get away with a 3" tube this might be a cool alternative if you can find a "clear" cap: http://www.tapplastics.com/shop/product.php?pid=274&
3" OD x 24" L w/ red cap is $ $6.75
Another cool feature would be some sort of quick-connect/release lanyard. I know that tube rides can get bouncy at times, so having an extra level of security would be good and prevent any downtime retrieving equipment (the rider is another story..)