I'd probably use the sharp edge of a file to create some "training" channels for the rush to sit in. Creating these small indentations gives the rush somewhere to sit until the next row snugs up against it. Apart from that, I suppose you could put some temporary tacks on the underside every few rows to keep them in place, then remove them when everything is filled in. I'd probably try filing some valleys in the top edge of the seat perimeter, though, first.