It's getting delivered today.
I went ahead with the 16"x36" because it gives the exact inside
volume [edited to correct a mistype here] I need, it's already black, and it looks thicker than the sonotubes I've seen. After shipping it's the same price I'd pay for a 16" or 18" tube from a local place. I'd have to throw away the remainder of a cut sonotube because I have nowhere to store it, so it was a wash. Plus there's no rush since the amp is still on backorder for a few more days.
It just happened that the Bascore tube they had in stock was a perfect fit into my project, so I bought it. I'll post pictures.
http://jonnythan.com/subwoofer <-- this is the site I'll be using to document the project. I have a WinISD screen cap and a picture of the driver up at the moment.