Did you try to maybe offset the angles of your current shapes and loft only the edges, leaving a empty shell in the centre.
Edit:
I tested this and it does not work, so i went back to the drawing board.
What I did was I made 4 sketches as seen below.
I then made an offset of each sketch as shown below
I lofted both sketches making two bodies.
I then subtracted the smaller one from the bigger one and got my final product.
I am still new to S3D and plan to purchase it one day.