gave this mod a go a few weeks ago. I sourced all the parts on ebay, all delivered for under $100. Used a stainless 4" bend, a 3" - 4" 90degree silicone bend, and a 4" hump hose:
I had this done today to my ute. Its not alot louder than the stock exhaust which is nice. I had seen other center replacement pipes for sale on here which were twin pipes straight through my centre muffler was a single inlet with twin outs. For me its loudest around 2000rpm (FG xr6t Auto). I'm happy with the outcome for my $70