First off, I suggest ditching the seperate feeding tub. You're not doing yourself or your snakes any favors there.
Seperate feeding tubs are old-school and proven counter productive over time.
Using a seperate feeding tub can:
A) lead to refusals because the snake is stressed from being moved
B) lead to regurgitation because you're moving/stressing a snake that just ate
C) increase the chance of you getting tagged by an animal in feed mode.
If you're worried about cage aggression/the snake associating opening the enclosure with food, don't be. I have always fed all of my snakes in their enclosures and have never even been struck at INSIDE the enclosure. I've been tagged OUTSIDE the enclosure though.
If you're worried about substrate ingestion, don't be. I've had hundreds upon hundreds of feedings and never an issue. Remember: they eat in the wild, nobody puts them in a clean little box, and they've survived thousands and thousands of years.
If you're worried you can throw a piece of cardboard or paper plate down in the enclosure ahead of feeding time, but I find they drag the prey off if it the vast majority of the time anyway.