UPVC soffit board- hollow vs foam?

[Crofter]

I need to replace the soffets, when I built the house I just used cheap ply to get the job done and three years down the line it's failing badly.

I think uPVC is probably the best option as once fitted I'll never need to think about them again. I'll likely go with a smooth finish dark grey or black.

 

Annoyingly, because I used 5mm ply last time, the support battens are all made for that thickness and it would be a massive, massive headache to change that (probably have to remove the gutters and fascias to get at the fixings. Yeah I wouldn't build it like that again). The board can be thicker in the middle but for about a 2" strip down either edge can't be any more than 5mm.

 

I can't find any off-the-shelf product that is that thin, but I was wondering about modifying a hollow or foam-core soffet board. The hollow ones already seem to have one thin edge, but they seem to mostly come in a wood grain finish.

 

What are the foam core ones like? If I stripped back one skin and the core, how thick is the remaining skin?