The heat shields aren't that expensive £45 each new long term you will wind up melting the plastic and the diffuser will look like nothing.
I think you cut the slash part of the tailpipe off to match the 200 exhaust there are plenty of 200 back boxes about as well then you know your good to go.
Look at this poor sods diffuser on gumtree he wants to swap back to a 197 diffuser
That's what yours will look like if you don't sort the heat shield properly sell your old diffuser and buy 2 heat shields or get another one with the shields fitted and sell the one you have on.