Specifically Terrafirma TF110 extended arches as I keep ripping the screws out. I can't complain too much as they're decent arches for the price but the vacuum-forming around the screw recesses makes the material very thin where you want a bit more robustness.
Thanks to Terrafirma answering my email I know the arches are LDPE (Low Density Polyethylene).
Work in progress - I haven't done the job yet but wanted to make a note of what I've bought.