One great way to keep substrate off the dead prey being fed (that's IF your snake will feed this openly) is to use a good-sized hide turned upside down. Many snakes like the sides around them for a sense of privacy, & with only the doorway for the snake to slide in through, it works extremely well to keep prey separated from substrate. Feeding in dim light & in evening hours will also promote success. If your snake is happy with drop feeding, or even with a slight tease, you could (gently & quietly*) put the upside-down hide with "dinner" near the doorway of the hide your snake is peeking out of- & hope he takes the hint. *Too much activity on your part can be an appetite-killer, so be as subtle as you can.