88chevy, without seeing the parts I'll take a stab at brainstorming and may just be stating what you already have thought,
So is the problem that the Mini pipe is smaller than 6 inches and bigger than 5 inches?
perhaps something like this in 6 inch diameter can be screwed to the top of your pipe and protect the gap?
Or if you prefer a spark arrester like I made, you might cut some sheet metal to cover the gap between the inner and outer walls. you may use a wall finisher donut that is 6"I.D and 9"O.D. from ace hardware?