Cant create surface or solid fillet - surface modelling - process help

[sandy.sempleYEZDM]

Im trying to create a part which is based on 3 surfaces, when i come to fillet the part it wont work as a surface or solid.

Can anyone advise me on how a part like this should be made to allow filleting? I feel like i dont understand the correct process in fusion/inventor for creating a part like this with surfaces. 

Im trying to get the part to look like the one below. 

[blandb]

Maybe you want to try and improve on this model? Looking at this in a wireframe view, there isn't really "wall thickness".

The side walls appear to just be a thin line:

[sandy.sempleYEZDM]

Thanks for the reply @blandb - I had another look at it over the weekend, I think my issue was that my lines used to create the surfaces at the corner were not actually coincadent, they were off by something like 0.002 mm, after sorting this, thickening and making my solid it seemed to allow for filleting.