Hey friends! Take a look on the figure below, here are two similar model with simple geometry, but when you take a closer look, you will notice there are a bit difference between this two models. As indicate in my picture, the ending face of the 1st model are normal to the adjacent face whereas for the 2nd model is not. Besides that, the transition of flanges at the fillet corner for the second model are not as perfect as the 1st one.

When handle this kind of model, you may apply simple features such as extrude->chamfer (for the flanges)>fillet (at the four corner)> and follow by shell to make it hollow. Somehow you may not get the desired outcome. The steps I have mentioned above is the technique being applied to create the 2nd model! To get the nice result as shown in the 1st model, you may try sheet metal feature if it is a sheet metal part, or else you can try another alternative which is using surfacing. Here are some simple steps for surfacing approach:
1. Draw a recrangular, extrude surface and fillet the 4 corner.

2. To create the flanges, you can use Ruled Surface. This is a function for you to create or extend surface base on the selected edges on existing geometry, with a reference plane/face, you can control the angle of the flanges! (there are some other options in this function such as “normal/tangent/perpendicular to surface” for you to create a face at certain direction.
3. How to handle the transition between two flanges at the fillet corner? I will use loft surface. To control the behaviour or direction of the lofted face, we need to create the guide curve. Just insert a 3D sketch and use a spline line to join the point of two flanges as shown in the picture below:

I want to make sure the lofted face must tangent to each connected flanges. To achive this, I will add a tangent relation between the spline line and the edge of the flange at each side:

4. When apply the loft surface funtion, define the edges of the flanges as loft profile (edges of geometry can be the loft profile besides sketches) , while the 3D spline curve and the opposite fillet edge will be the guide curve. Take a look at the figure below:

5. To complete the loft surface for another 3 corner, i will rather use circular pattern! But this is a rectangular shape geometry! How?:

Just create a dummy circular surface body at the center of the rectangular. When you define the direction of circular pattern, you can either pick the circular edge/face / axis to chieve this! What you need to do after this is just make the dummy circular surface body hidden!
6. Finally, use Knit Surface to combine all the pieces (remember, you are working with surface body, not solid!) then, turn this surface body into solid by using Thicken option. Here, you can define the thickness of the solid and also the direction to add in the thickness!
Hope you get some idea from this sharing~ ^_^