Bringing this up once again. I've been smoking my butts on the top rack and there is plenty of room to put a pan underneath it to catch any drippings before they get to the bottom of the SI. Is there any reason not to do this...seems it would keep the bottom a lot cleaner.
I am quite late in posting on this, but my solution to this is simply putting small loaf pans on BOTH sides of the firebox. This way there is no interference with smoke patterns or heat sink issues above the box. I put just a small amount of Apple juice since I brine my Butts and Briskets and there is plenty of moisture. These two pans don't catch everything, but they catch a lot and cleanup is so much easier. The pans are easy to remove and the high sides make safe removal hot hot liquid a bit easier. I can essentially just pull out the foil bottom layer with no issue or mess, and no wiping of the bottom is required.
I usually take the juice from one pan and refrigerate it while the meat is resting. When it's time to pull/cut the meat, I take off the fat layer and microwave the juice and add a bit to the top of the pulled/sliced meat and stir it around a bit. The meat doesn't really need it since its moist already but my family thinks the added moisture makes it even that much better. Finally, I just give the pans a quick wash and reuse again and again.