"I can do that. Frisk, do you mind guiding the hooks until I get back?"

Whatever happened in the previous room happened in the inverted room so it would make more sense to go back and place the crate in there. Even if it didn't, the crate would still be above the door, leaving it able to be opened.

So, she'll return to the other floor with Kanji and Sephiroth, but then walk back into the room to start moving one crate over to the door.