Another chimney brush question

Hey all, current chimney is clay lined 6x10.5 ID. I know, I know, I need to get a liner but I cant right now. What size chimney brush should a use for this? Lowes and Tractor Supply have an 8x8 square and 6" Round. Im sure I could order a 6x10 online but I need to get this swept asap. What size would be best out of the two above? Would the 8x8 even fit? My concern with the 6" round is that I wont be able to get into the corners very well.

Look at the soot eater. It can be made to fit what you have now and then cut down to what your liner is. I love mine.

Here ya go , custom sizes, including rectangle....
http://www.caseybrush.com/customchimney.html