This requires welding skills or access to a welder and knowledge of how to weld.
I needed a spark arrestor for use in a national forrest, but didn't have a stainless screen insert for my Dr. D, so I made one from an old Supertrapp (ST). I took the Dr. D end cap with me to the auto parts store and found an exhaust pipe adaptor that would fit over the end pipe (about 1 5/8" or 1 7/8", can't remember), but it fit over the end tube with little play. I also got a 1/4" bolt about 6" long. Then I got the correct U bolt clamp for that size pipe.
At home, I welded one old ST diffuser plate onto the end of the the adaptor, making sure I had the correct side out. Sliding the adaptor pipe onto the end pipe, I located a place where I drilled holes thru both sides of the adaptor so I could slide a cross bar in to hold my bolt. The bolt holds the end cap and diffuser plates on. I made sure there was also enough bolt length to hold at least 12 diffuser plates onto the end and still bolt the end cap on.
With the cross bar in place, I tack welded the bolt to the cross bar making sure it was centered in the adaptor tube. Once I was happy with the locations of everything, I finished all welding, and took a hack saw and split the end that would slide onto the stock pipe end in 4 places, and bolted the spark arrestor onto the muffler exit. While it's not pretty, it does the job and no ranger can poke a stick thru it. With 12 plates, its slightly quiter than stock at slow speeds, but open it up, and it sounds great, though not sure how loud it is since it has not been tested. And it didn't kill power anywhere in the power band... I can still loft the front wheel in 4th on my YZ250F. Anyway, it works for me, and thought I'd pass along.
Note: USFS approved spark arrestors are required for use on public lands. While this mod might be effective, it may not pass an inspection.