Here are some patches that I made for a Reynolds Contempora that had developed a series of cracks throughout the body of the instrument.  The first picture is on the mouthpipe just before the main tuning slide.  The offending crack extended all the way to the tip of the brace.  To make sure the crack was completely covered, I cut a notch in the patch so it would fit around the brace.  This also serves to minimize the appearance of the patch.  The line of reflection is uninterrupted except at the ends of the patch, which are the only giveaways that it is there. 

These show a couple of patches that were placed in the thumb valve area and details of each one.  The one to the left of the valve in the first picture is my favorite, by far.  I minimized its appearance by butting it up against the ferrule and making its edge follow the line of reflection on that piece of tubing.  Patches have to be meticulously filed to shape to ensure proper fit.  The patch on the right clearly shows the need to allow for bending clearance.  The knuckle on the valve takes a sharp bend which would cause a rectangular piece of metal to buckle when bent, so the patch is cut into a shield shape, which takes the curve well.  It is then burnished to fit.  The flat end is butted up to the ferrule to minimize its appearance, although, the necessity of the shield shape makes this patch harder to conceal.  A patch can also be made of scrap tubing if one happens to have some with the proper bend handy.