Plastic Cornice fitting

Old Snowdrop

Having carried out some repairs to the rotten wood and had to take out various panels I am now trying to replace the plastic cornice, but this is supplied in a U shape and I need it in a V shape. how do I fit this so it fits the right angle profile.  I was
told to staple / pin it to the surface but it would appear it is trying to return to the U shape which will make it wavey. I hasd thought of glueing it along both edges?

Any suggestions?