I develop my 8 x 20 negatives in a 16 x 20 drum, using Pyrocat-HD by hand. Machine rotation caused too many problems. My solution to perfect negatives every time (using some of Sandy's tips):
I have crafted/cut an acetate sleeve that is inserted into and covers all of the ribs in the drum.
I only develop one sheet of 8 x 20 at a time.
I use 2 liters of fresh developer per sheet.
Rotation is by hand and slow at 4-5 revolutions per minute, with reversal of direction and gentle tilting.