I've just received and installed one of Tinman Tim's new tool tube mounts, referenced in the threads: It's Coming and Installed Tinman Racks. This is an elegant solution to the tool tube mounting problem.
The aluminum mounting plate looks like this:
Bolted up to the bike frame, and with the tube attached, it looks like this:
The bolts with the big washers are carriage bolts which secure the plate to the frame. I found it helped to tap the carriage bolts into the aluminum plate to seat their square parts so they wouldn't turn. Though the included instruction sheet didn't mention them, Tim included two larger rubber washers too, which I put between the plate and the frame. Perhaps another set between the outer washer and the frame would also be a good idea.
This is what it looks like with the seat and fender reinstalled (cap left off tool tube to see the thing better - I'll put it on, I promise):
In addition to not knowing for sure what to do with the big rubber washer, I had two other minor issues, about which I've PM'd Tim directly. They were:
1) When mounting the tube, if the inlcuded carriage bolt head goes outside (against the black plastic) it's impossible to tighten the nylock because the head turns. If the head goes inside (against the aluminum), the bolt end sticks out just enough to interfere with replacing the tube cap. I solved this problem with a hacksaw. The included bolts are 1/4 - 20 x 3/4. 5/8" long would be perfect. Perhaps better yet would be some 1/4 - 20 x 5/8 hex bolts, with the head on the outside and the nut on the inside (would require washers too). Even more elegant, at a cost of about 3¢ more, each might be Socket button head bolts. (I've dealt with Bolt Depot some and they seem to be a good outfit.) I might change mine out to these, and would probably go metric as those are the tools we use on everything else.
2) The plate does make reinstalling the left seat bolt a bit challenging (and yes, I've read the threads about modifying that system, but haven't done it yet). I applied some gentle persuasion with a ball peen hammer to the top of the plate at that spot to make said installation easier.
These things are a great idea, and I'm glad I've got one! Actually two, have another for the other TW we have. I hadn't figured out what I considered to be a good, sturdy way to mount a tool tube, but this appears to be it. I haven't ride-tested the thing yet (love those Colorado spring snows, but as every good Coloradoan will say, "We need the moisture") but it certainly seems quite solid. If any issues arise after riding I'll come back here and let y'all know.