I'd probably go 4231, play a flat back 4 with a holding anchor man in front of them. Not sure who would play the holding role though, Thomas runs around a lot but not sure he is disciplined or strong enough to play this, only other option is maybe Finch and that isn't ideal. Not convinced Jackson would be any good out wide either. We're short on midfielders and have 5 strikers so I'm not sure how clever it is to play 5 in midfield with 1 striker.
Would have to be fleck as he word just drop back there naturally anyway. He's nearly always the deepest midfielder, as it's the only place he gets time on the ball.