If you plan too ride the woods all the time, might pay to get a proper arrester. However if you only want to ride the woods occasionally and don't want to fork out a lot of dollars, go to a supply store and get some stainless mesh ( 1-2mm, like on fly screen door). Fold it over the end of the exhaust and use a hose clamp to hold it in place. Join 2 clamps together if you cant get one big enough. Put some duct tape around muffler first so you dont scratch it. Doesnt look to flash, but it does the job. You might be able to take the end of your muffler and attach it inside somehow. Thats all we do when we ride a cross country in a forestry block. If the mesh isnt stainless it will burn out in a few minutes.
I got a 04 crf250, done about 150 hrs & wouldnt go after 6hr crosscountry race. Valves & valve seats were nackered. Did valve inserts( exhaust ports) & new valves, piston & rings while I was at it, now its like a new bike. 2 or 3 kicks cold, 1 kick hot, sweet as.