Filling gaps in window beading

[Newington]

I've attached a wooden baton to the beading of a windowframe in order to hang a blind. The window is an awkward shape and I've been left with gaps at both ends, where the wooden baton meets the sloping wall, and some of the pre-existing beading is missing. There's a small cavity behind each gap, as shown in the below photographs, which ideally I'd like to leave unfilled. Can somebody pls advise on how best to fill these so the window looks "squared off"? Cutting bits of wood to fill the spaces is not an option, so I'm wondering if foam filler might be a good solution? Thanks

[Grendel]

Don't use expanding foam. It will expand where you don't want it , require cutting to suit afterwards and even after painting will look a mess.
Looking at your pictures I would say the easiest option would be to cut an additional piece of timber , mdf will be fine for a blind , and fix it on the face of what you have. Have it deeper than what you have there , say four inches or more , to cover both the squared off piece and the " gap" and cut the ends to match the slope of the ceiling. Caulke if needed and paint and fix your blind to that.