When I make a round shield:

• I first cut a two foot square piece of plywood (1/2 inch, 9-ply). I draw a line through two opposing corners, and then inscribe the circle. I then mark two points, each one inch outside of the circle on the diagonal. I then adjust the circle to take in the points, leaving a circle with two slight bulges.
• I then attach a steel edge. The standards from standard and bracket shelving work well. Aluminum can be used if weight is a factor. I attach it at eight to twelve inch intervals with U-shaped brackets made of plumber's tape, bolted through the shield an inch or so inside of the rim.
• I then cover the edging with a section of heater hose, or a bicycle tire, if cost is a concern, which I overlap a few inches, then super-glue and sometimes stitch with wire.
• I set the straps by placing my forearm on the shield so that it follows the diameter with the extra two inches, placing my elbow about two inches from one edge. Since I use punch blocks, this gives me several extra inches on the punching edge without the hindrance to vision and extra weight that a round of that size would have. The back strap will cross my arm an inch or so in front of the bend of the elbow. The other crosses my palm.
• I prefer soft, heavy leather straps, the front being a little loose, and the back with an adjustable buckle, which I also keep a little loose. I use 1/4-inch bolts, with washers and locking nuts. The bolts should be two to four inches in either side of the arm and hand. When I grip the front strap, I always hold as close to the top bolt as possible, so that when my hand tightens, the top of the shield is pulled in towards me, thereby assisting me in holding it at the correct angle. Other people prefer a solid handle in the front. If you do, I suggest you mount it so that it is tilted, with the top end shorter than the bottom end.