TBH I think you'd be best off without a pad. If it's improvised it might well slip and if that happens you won't stand a chance

I often use a pair of jods with a Clarino (fake suede stuff) full seat for bareback, these are the most comfy & grippy things to wear IME

This time of year is interesting for bareback with coat changing - you might find your jods are hairier than your horse when you're done