Feed in the enclosure.
I have never had issues with a snake associating opening the tub as food time. It is actually more dangerous to feed out of the enclosure. You have a higher chance of getting bit because the snake is still in hunting-food mode after its eaten. I'd hate to stick my hand in to pick up a snake to move back to their enclosure while they're still in that mode. I like to leave my animals alone after a meal so they can digest in peace.
And a little ingested substrate isn't going to kill or hurt your animal. Impaction via substrate is pretty rare.
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