I have posted some of my tube items @ vendors, under "mega tube". I do carry a third size, not the 2 1/2" diameter, I just couldn't see enough use for that.
I carry a 3 1/4" diameter tube w/ a longer (taller?) cap than the standard agri model. (12 1/4" instead of 11 5/8") I have a few left, probably won't restock, not that great a demand. The only unique feature they seem to have is the ability to carry most glass bottled beverages, say, 750 ml.
I located the original manufacturer of the manual holders- he designed, developed, manufactures, and wholesales to tractor makers worldwide. I'm a tiny portion of his business, but he likes my business, so he sells to me. We are currently developing a parallel product for another market niche.
My large tubes are the result of my taking a chance, and putting up a few thousand dollars for a discrete production run, and shipping it to the states. Had I ordered 10,000 units, my cost would have been cheaper, but my quantities were quite a bit less.
Some may find the feature article in this month's Wired magazine quite interesting- It discusses the very new opportunities available thru the intersection of the internet, cheaper production tooling for the small shop, and the willingness and ability of chinese factories to produce small runs for niche markets. There is a car company in Mass releasing an open source street legal rally car- imagine producing and selling the ultimate dual sport?
So let me ask, what would you like to see in the tube? I've considered deleting the plastic brackets, specing stainless bands; sturdier, can possibly rotate independent of one another, perhaps negating the need for intermediate tube to bike bracket adapters. Quarter turn caps? Integral locks? Anything can be done, quantity is the key (think 500-1000 units). The tubes are a great product, everybody has there own ideas how to best use them, and some small improvement to make them that much better.