I'd also try a mouse instead of a rat pup. A hopper would be a little more durable.

Also while it's not ideal to give two feeders sometimes it's easier to get something like a large fuzzy pushed far enough back so the snake can't spit it out easily, then when the snake starts chewing or swallowing, you can train feed a second fuzzy or a small hopper behind it. The snake gets a decent sized meal and you can wait a few more days between feedings to reduce stress on both of you.