Need A Suggestion on a Mechanical Problem

The truss connector blocks are flat on the base, because they are normally attached to a flat surface.  In this case, I am attaching to a 20" OD round tube.  That is close to flat, but not quite.  Where the blocks screw to a surface, there is going to be about 1/16" gap on either side.  I need some spacer in there that will prevent rocking, and is about 1.25" long and perhaps .375" wide.  This is a bit small to be cutting from steel or aluminum (at least with what I can find sitting around on a weekend).  Any suggestions for something that I might find in a hardware store that is stiff enough that it won't bend much, but can be cut with tin snips?

UPDATE: A conversation with my wife about the subject suggests a solution to try tomorrow: take some of the pieces of aluminum that are too thick (.125") and sand them to a wedge on the sander.  This will produce wedges that can be milled to the required dimensions.  Once to size, I can tape them to the back of the blocks, then drill them through the existing holes in the blocks.

UPDATE 2: Sanding aluminum to a wedge is surprisingly slow.  Does anyone have an idea of some common thin wedge shaped aluminum or steel that I might find in a hardware store?  Are there shims that are wedge shaped?

This might do the trick: they are plastic wedges intended for construction, and self-adhering.  What the various dimensions are describing isn't terribly clear, but I suspect that I might be able to run these through the band saw to cut them into the wedges that I need, and then mill them to the precise dimensions that I need.  Then use the adhesive to stick them on the bottom of the blocks before drilling through the existing hole in the block through the wedge.

UPDATE 3: One reader suggested the wedges used for repairing axe handles.  These usually come as a tiny piece of steel (for exerting force) and wood.  I was hoping to use metal or plastic to reduce the problem of rot.

One possibility is to use a piece of aluminum C-channel; cut down the legs to 1/16" of an inch, and that provides something that prevents wobble, and provides a flat base.  I tried to mill a C-channel from an existing 1/8" sheet of aluminum, and while it looked great, it was very slow.  It probably makes more sense to start with a piece of aluminum C-channel, cut it to the needed length (1.25"), cut the legs down to 1/8" inch on the band saw, then mill them to 1/16" inch on the vertical mill with a fly cutter.  The problem is that I don't have any 3" wide C-channel lying around, so I will probably have to wait until Monday to get it, which means waiting on this rebuild until next weekend.  I wish someone made acetal C-channel-- lighter, and it doesn't need to be spectacularly stiff or strong.

UPDATE 4: Final solution, I think.  Using x² + y² = r², I compute that for a 20.5" OD tube, the drop over a distance of 1.45" in the x-direction means a .103" drop in the y-direction. What I will look for is some1/10" or 1/8" round rod, and I will then epoxy 1.25" lengths of this on to the tube where the two sides of where the block will go.  This will give a flat surface upon which it will mate.