How to transform a solid with some lines

hello,

I try to transform my solid with the cross just under the solid like the picture attached. I don’t understand how I can do. If someone can help me I will be very enjoy.
thanks a lot in advance

deformation.3dm (2.5 MB)

Hello - does this file help at all?

deformation_Maybe.3dm (215.1 KB)

-Pascal

hello thanks but on the back side I need that is not a plane form. I need that the other side is the same at the front side.
Could you please if you have time explain the difference steps?

thanks a lot ,
best,

Nicolas

please find here a test I made but I don’t arrive to close the corner…
thanks a lot,

Nicolasplat.3dm (2.6 MB)

@pascal Golay

wich tool are you use to make a corner chamfer because when I do this tool I have this (please look this picture)