Hi Fredrick,
Your process is correct only I had outer and inner as solids ie with capped ends.
The fact you showed the bottom thickness as 4mm and the side walls as 3mm was the reason for making an inner.
So the 2 lines at the bottom are showing the bottom angle and then offset 4mm this line is used to Boolean difference the inner to give the bottom 4mm thickness.
Boolean difference inner from outer and you have your part.
Hope that’s helpful
Cheers Barry