I had broken an existing holder for my tail light on my bicycle. So I needed away to repair it, so as always I looked to the internet. I found a site that talked about building recumbent bicycles. They had a good article on building clamps for different requirements such as connecting the front idler, the seat, the jackshaft, the remote steering arm and the remote steering tube. They did not want to braise these items directly to the frame because unless your bike is designed perfectly, they will probably have to be moved and you might need to do some redesigning later. It looks like it would be easier to just unclamp a bracket and move it.
To build one, measure the outside diameter (OD) of the tube you want to clamp to. Find another tube I use a piece of pipe with the same Inside Diameter (ID). Cut off about 3/4" section. Get some 1/4x20 or #10 threaded 1" long coupling nuts (available at hardware stores). File the side of the nut to match the contour of the tube. Braise a long nut on each side of the tube. Then you can braise on any brackets, attachment pins, nuts, or tubing to the clamp.
After you have finished braising the nuts and whatever on, cut the clamp in half. Using a tap, clean the threads out on one side, and drill out a clearance hole for you bolts on the other. I used cap socket head bolts. They really give the job a finished look. You should file the brackets so that there is a small gap between the cut ends of the nut so that the clamp will be tight enough.
Here are some examples of the clamps that you can build. They do not necessarily need to be on a bicycle it could be used on all types of machines.