I had to cut out a small section underneath when fitting the towbar, and it seems to be similar material to plastic guttering. The rear bumper is quite easy to take off anyway so you might have more luck from the inside; it's only a single skin. Two hex head screws each side of the tailgate opening and a couple of fixings in each arch and along the bottom, then it's just a question of pulling loose from the friction clips at each side and below each light cluster.
But.... I don't think deforming plastic without a mould will ever yield perfect results.