I have been very impressed with Hillhaters use of a bikes handlebar stem as a mount option, and I am trying to figure out how hard/easy it would be. AJ pointed out that stems are available with an adjustable angle, and that solves several problems for me. Niagra bicycle parts has 38 pages of stem selections, so I am collating the adjustable-angle stem options available to me.
The vertical post that inserts into the steer-tube of a bike is the quill (like the quill of a feather-pen being dipped in ink). The length from the center of the quill-clamp to the center of the handlebar-clamp is often referred to as the reach.
The quill-clamp inside-diameters (ID) are either not listed, or are listed as 1-1/8" (1.125", 28.57mm)Bar-clamp ID's
listed are in millimeters and are
22.2, 25.4, 26.0, 31.8Reach
are also in mm, and are
80, 90, 95, 100, 105, 110, 120, 125
If you wanted to use a fixed stem, just like Hillhater, they also list the "rise", which can be zero (90-degrees in relation to the quill) up to a steep angle. Fixed stems also present an intriguing option. The bar-clamp end often has a removable cap, which might allow the stem to be attached to a bike-frame seat-tube, rather than the removable seat-post (though the biggest bar-clamp ID listed is 31.8mm).
Some stems for downhill (DH), Dirt-Jumper (DJ), and BMX bikes are short and quite strong, and many of them have zero rise, such as the pic below.
There are various adapter-sleeves called seat-post shims, split-bushings, bore-reducers, etc. They may allow a certain stem to be used with a particular diameter of tubing. I have made these in the past, and my favorite materials are a section of brass plumbing pipe, aluminum crutch-tube, or electrical conduit.
The original diameter of the pipe is not so important as the wall-thickness. You can spin the pipe by some method and file/sand the OD until you achieve the proper wall-thickness, then chop to length, and slice one side to allow it to spread out to a useable diameter. I have also cut the pipe into two half-sections when appropriate.http://problemsolversbike.com/index.php/products/seatpost_shimshttp://www.niagaracycle.com/index.php?cPath=108_188