Quick question on fitting window boards before install tomorrow...!

[Sorbustree123]

What would be a good product to use to level the surface and fill holes before fitting MDF window boards 25mm thick? Guessing some adhesive might be good after levelling? Any advice very welcome. Last minute rush to get sorted tomorrow before plasterer starts skimming room first thing on Saturday. Thanks in advance!

If I was fitting a window board on there, you wouldn’t need to do anything it looks solid enough. It doesn’t need to be dead level as you level the window board when fitting.

How do you level the window board when fitting? Many thanks!