I also have a BA pad , and use a mummy bag , I find that I slde off to the sides , and if I lay on my back , my arms fall to the tent floor, so I usually put my riding pants on one side of me and my jacket on the other side which sort of extends the width of the pad and keeps me centered on it .
Not the best solution but it helps .
12 Vstrom 1K
09 KLR 650 (BLACK)
09 Yamaha WR450F (street legal )
04 prairie 700 (plow duty)