If you want the walls to be 1.2m thick horizontally and the base 1.2m thick measured vertically:
Draw your external profile. Move with copy vertically with the base up 1.2m and move with copy horizontally with the curved wall by 1.2m. Join them up.
If you want the walls 1.2 thick measured at their thinnest point, like radially if your curve is an arc, do this:
Draw your outside profile. Make it a closed shape then use the inset tool with 1.2. Delete the outside part at the top or extend the inside, depending on what shape you want the lip.