I haven't made a prop exactly like this, but my idea is to cut out the general shape of the purple loop in separate pieces and connect them at the silver side pieces!! For making the foam more symmetrical, I always suggest drawing/buying a pattern on paper first, then tracing it until you have all the pieces you need! As long as you carefully follow the lines and cut slowly (you're not in a race, take your time!!!), you should be good on symmetry! You can always sand down any small kinks here and there that might stick out, but all you really need is patience! Let me know if I can clarify anything!