Heres a pic of mine with a collins moongoose with a 3,1/2" tailpipe, best size, have had a 3" & 4" on and this is the best fit to bumper hole size, and not to noisey compared to the 4".
You used to be able to buy just the tailpipe section from Scorpion with the rolled out end, maybe buy one of Pauls 2nd hand ones and weld the 3,1/2 " section onto the back box, cheap exhaust then.
If Scorpion no longer do it i am sure ebay has these stainless steel sections with the rolled out tail pipe around the 20 quid mark last time i checked.