Glued up the pieces temporarily with small drops of superglue, and sanded everything nice, square and flush using my disk sander.
Above, using a file to clean up the end parts so the cross piece will sit level.
I used an old machinist’s trick by marking the parts to be filed with a (red at the moment, it hides the blood) Sharpie permanent marker, so I can see where I still need to do some more filing.
And above here, all the parts superglued on.
Next, I need to measure and mark the hole locations, and then drill them.