Righto, some experimentation later ...! I've just tried all three nappies on her, each stuffed differently.
One had the large part of the WNOS insert stuffed into the pocket. This was the worst fit. Needed a middle rise setting and left the middle waist popper exposed. Gaps around the legs that you could have driven a car through.
One has a FB MF insert and an EP hemp booster stuffed into it. Just a bad fit.
Middle rise setting again and also left the middle waist popper exposed.
She's now in the last nappy, stuffed with a EP hemp booster and a MF cloth. This fits best on the lowest rise setting and the waist tabs now meet (no poppers exposed), but don't crossover. This is still going to leak, I fear. Plus, it's not going to last very long as there's not a lot of stuffing in it ...
I'm going to have to admit defeat unless someone has a brainwave.
If these have to go, DH will kill me - they're practically new. And his love for BumGenius will be vindicated.