I would go about this in a way where unless you're able to cut super accurate on a band saw or cut off saw is perfect you're gonna end up wasting about 45% of your stock.
1) Start with 2" od 1" id tubing and part it of to the lengths you need.
2) Cut these in half two make two Cs, this is where if you're perfect you can get two if not you only get one out of this section of tube.
3)Mill a profile of this part in soft jaws so it's held well in my vise and face it till it's the perfect half ring you need, measuring this will be a pain but if you mill your soft jaws right you'll know where you need to be.
4)From there I'd drill the holes, 2 for the one 4 for the flat part.
5)use a Different vise and turn the parts of so they're sitting on the side you face milled. I'd then use a 2 flute end mill to plunge the location of where the bolt heads will be. And for the flat ended parts i'd face them down.
Sorry if this is confusing if you need clarification on anything feel free to ask.