Square Cross Section: Step 3

Once we have the edges constructed, we can start to build the sides of the solid. The metaphor is to imaging stretching on edge linearly to the other edge. Another edge is to consider "mixing" the two edges linearly. The command to do this side is Surface[u*1 + (1 - u)*t, u*0 + (1 - u)*0, u*0 + (1 - u)*f(t), u, 0, 1, t, 1, 5]