I use a 5-gal bucket in which I drilled a hole in the bottom where the rod can pass through. I then take off the lower section of pipe and tape the bucket to the pipe with painter's tape. With my wife providing additional support, only very little soot rains down through the hole.I'm assuming you take your pipe off the stove and clean up from the bottom? How do you keep that from pouring mess all over? I had quite a mess with the brush and had to use the shop vac every few minutes. I tried the bag trick but was working so hard to get the oversized brush unstuck and to move up and down that the bags tore.
It is a little messy, but if you go in through the bypass hole you don't have the opportunity to vacuum behind the cat.