Not an easy one (at least not on my train of thought). Maybe someone else has another way that could work out better. Without dimensions I guessed at how some of the form should be from your pictures. You should change the plane angle to alter the depth of the angled sides.
The side clearance angles are chagned from the extrude the way it was to a draft after the top was done. This was to maintain the part width but I really do not know what you have to hold.
I cleaned up the file a bit but I have to post it here as an Alibre step file.
This means you will need to import an alibre.stp file to read it. This stp file is not the type that can be read by other software but if you need it in a format like that I could send it. I am only assuming the version of Alibre you have will open this file so let me know.
The biggest trouble I had is the corner radii where the transition occurs. Not an easy place to add fillets but it should not matter for machining. If you are using a 3/4 ball endmill then the smallest radius it would machine is a 3/8 radius anyhow.