It is difficult to cut a straight line by hand with thin cut off wheel in an angle head grinder. I had an angle head grinder and wanted to make a mount for it to convert it to a cut off saw. I know you can buy a mount at Harbor Freight Tools, but I have a bias against most of their tools and wanted to make my own. I also have a welder. I used scraps of steel I already had.
Step 1: Basic piece for a start
A friend gave me some scrap pieces of steel. One of these was a piece of 1 inch square tubing about 22 inches long with a 90 degree bend at one end. For a long time I had no idea how I would make use of this piece. If you choose to make a saw like mine, you probably will not have a piece like I had, but you can use other things. You could even weld a couple of pieces of square tubing together to make an "L" shaped piece.
Step 2: First step
I cut about 9 1/2 inches from the straight end of the square tubing and welded it in a "T" to the end of the 90 degree curve. See the Sketch up drawing to see how the parts are positioned for welding the two pieces of square tubing. Then I welded two door hinges to the 9 1/2 inch piece.
Later I added a magnet from a hard drive. It makes a convenient way to store the bolts for securing the grinder to the mount attachment. The magnet is visible in the lower right of the picture.