Beefed up nylon bearing, to combat the bobbin popping out under increased tension.
I've rotated the opening towards the back 10° and made the flexible bit both solid and wider. Considering increasing the radius as well, but I want to see if this works before I start altering the geometry too much.
Edit: So far, so good. PLA doesn't seem to be the best choice, but it's working. It needed a touch of Super Lube to combat a nasty squeal, but the friction, even under the increased tension of what I'm told is Navajo plying, isn't causing any damage to the plastic. It is also holding the bobbin under greater loads, exactly as intended.
I presume one would be better off using a highly heat- and wear-resistant plastic, but my prototype is in PLA and has held up to several hours of spinning, so far.