Fill an arch opening

[Pietrob]

Hi everyone,

I'm new to archicad I hope you can help me
I created a Mansory arch type opening with a brick vault. Now I have to close it with a new wall so that I have it as a new element in the renovation.

I tried with operations on solids but I don't get the opening as an operator.

In the attached image you can see what I actually have, the red lines would be the wall I have to design.

I tried with different methods but I can't make it, can you give me some advice please?
Thanks
Pietro