Bay window leak and replacing rotted plywood sill

[CornishJon]

Hi all,

I'm new here and would appreciate any advice..

I have a bay window (this style might be termed bow window).

I recently found it had been leaking for some time and completely rotted the plywood underneath the plastic interior sill (no idea how long it had been going on for!). I removed it all, spent a very long time diagnosing the ingress and found the cause was poor sealing on the outside (I bought house 18 months ago - windows were installed 5 years ago).

Anyway, I pretty sure I've sorted the leaking by redoing the sealant... but i'm still nervous to put it all back (including new plywood) in case there is any additional small leak or condensation.

My question is A. Could I, as an added measure use waterproof expanding foam under the window (in the small gap between window and sill) ? see pic.

B. Can I use something other than plywood for the sill, something more waterproof? I was wondering if I could use a strong insulating board or is that stupid? There is a thin plastic sill that goes on top it's just this bit underneath that I'm not sure on what to use.

Thanks in advance for any help