Hi Mal - if it's flat on one side and straight, you could build that by constructing one initial side profile like so:



Then offset that curve with "Cap ends" enabled to make this thickened closed curve result:



That can now be extruded (it may be better to put your profile in the middle and use "Both sides" mode for the extrusion for easy symmetry about the center axis line):



Then for the corner areas, pick these little edges here:



And use Fillet on them to round those off:




A different approach could be to do booleans or trimming to get a base surface to start with, if you went that way then you would want to use the Construct > Offset > Shell command thicken that surface into a solid, and if you went that way you probably would cut the base shape with a curve from the Top view and you could put rounded corners in that cutting curve so you wouldn't need to round the corners off at the end like above.

But the above way is how you could do it more from scratch directly rather than cutting the base object up. Either way is ok to use though really.

Let me know if that does not match what you were asking.

And by the way your final result is looking great there!

- Michael