You could use a garbage can lid for a mold for a round shield. Use modeling clay to cover the handle and make the lid smooth.
You could also use a saucer sled for a mold if you want a really big shield. If you use one of these, you can get a smoother front surface by molding the shield on the side of the saucer you sit on. Lay the first layer of strips radiating outward from the center
- Lay the next layer of strips at right angles to the first.
- Alternate like that as you build up a half-dozen or so layers.
- After it dries, pop it off the mold and trim the edges.
- Lay more strips around the edge to give it a finished look.
- Attach the arm straps of leather, rope or cloth by cutting slots through the shield and hold them in place using staples, glue or brass fold-over brads.
- Lay a few more strips of paper mache over the straps on the front side to cover them.
- Glue cardboard cut-outs to the front surface to create decorative details.
- Let that dry and decorate it with paint.
Don't rush the project. It needs to dry well before you paint it. It might take a week to do it up right.
See the link for some authentic round Roman shield designs.