Hi there. So this is what I did.
I always look for the simplest primitive to start with, in this case an extruded rectangle
From the Top (right side in pic) draw a rectangle which encompasses the shape. Use the offset tool to offset 3 sides by the material thickness, I used 3mm
Now extrude your ‘walls’ …
By placing construction planes on either long wall you can draw the geometry needed for you cutout shape
Cutout either side then go around and finish by filleting edges
This is only one way to ‘skin the proverbial cat’ and thats the beauty of direct modelling, there is no right or wrong way … only simple and difficult.
Hope this is of some use,