remove material between yellow lines where the green arrow is. the only thing to be careful of is the depth of the threads when done noted by red arrows. you can either deepen the treads by drill and re tap the same size or just shorten the screw. you may not have to do either, just be ready. i would try to remove the same from both sides just for looks but you could just remove material from the smaller or cap piece. it will work the same but i am just picky.
my sugestion for cutting if you don't have a mill handy would be a normal hack saw and then file to a flat smooth finish. alluminum cuts super easy and you may be able to remove enough with just a file. to do a good job you will need a vise just be sure to protect your clamp with some thin wood shims. vise jaws and alluminum don't mix. when your done some fine sand paper and a can of mothers polish and no one will ever know you did it.