Seeking perspective on cat intro with one hesitant resident (NM -> 2 NF household)

HI all! My partner and I are two weeks in to introducing my cat (NM, ~8 years old) to her two sister cats (NF, ~4 years old). We've been following the Jackson Galaxy method. My boy is mostly in his basecamp (spare room) with a towel under the door to prevent eye contact (tied to command hooks on the door), and are doing some site swapping. The one hiccup we have been running into is that one of the girls has had bouts of aggression towards the new cat. Most of the time she's chill, but sometimes she'll get really determined to pull the towel (or any other barrier we've tried so far) out from under the door. Once she gets enough of a glimpse of the new cat, she goes full Rambo on the door until one of us throws a towel over her and puts her somewhere else to cool off. The other girl seems a lot more OK with the new situation, and the boy is comfortable in his basecamp and doesn't seem too freaked when he catches sight of the girls (unless the one is trying to go after him).

Has anyone has been in a similar situation before? I'm hoping to get suggestions for other things we could be doing. Of course, this is still relatively new and we're both confident that things will resolve with time. One obvious thing we are working on is a barrier under the door that the aggro girl can't remove. But are there some things we can do with her that might make her feel less threatened / less inclined to "seek and destroy"? When she's distracted by food, she doesn't mind having the barrier lifted, and they all have no problem with each other's scent. We have wondered if having the new cat hidden from her (and the spare room sealed off) is part of why she gets aggressive. At some point soon we are hoping to put up a transparent barrier (thick plastic sheet) so that the boy and the mellow girl at least can take a look at each other while we distract them with treats and play. Would some kind of controlled eyesight activity help with the aggro girl, or is that way beyond what the situation warrants right now? I'm hoping to avoid getting stalled out, where the intro isn't moving forward for the one girl, without pushing too much and regressing.