First using a large face mill, take a cleaning cut off the top.
Next using a drill that very nearly matches the size of the clearance area around the bolt holes, drill down to the depth of the flange plus whatever added depth is needed to leave a chamfer on the bolt hole when the part is finally finished. Then drill the bolt clearance holes.
Next using an end mill large and heavy enough to cover the entire expanse above the flange, go once around the periphery and remove all the stock in a single pass leaving about ten thou for finishing. The large end mill will take care of finishing the top of the flange. The stock at this point is rigid enough for this, though your machine will also have to be rigid enough. Most any 40 taper should handle this.
Next chamfer the top of the flange. (Stock must be high enough above vise jaws for this.)
Now flip the stock on edge and using a tapered end mill in a single pass, (maybe two depending) cut one angle. Flip the part over and cut the other. We will assume the width of the stock was brought to size before starting. Flipping the part twice will not be that difficult as it is still an easily handled piece of stock and little care needs to be taken in it's lateral position as we're not creating any location specific part feature at this stage. Just an angle over the entire part. (I prefer this strategy over ball end mills and scalloped cuts.) Though in a part like this, large, intentional scallops in the slanted area might look pretty cool.
Flip the part back to right side up and now all you have left is a finish pass with whatever size end mill will make it into the bolt clearance areas, and using an appropriate corner rounding end mill, go once more around the periphery, some of it 3 axis simultaneous, and you're done. (The flange top may have to be cut with the finishing tool, as the large tool that cleared the area will certainly leave a small uncut area around the bolt holes. Whether you could control the depth of the finisher in relation to the rough cut would have to be seen.)
Personally I would have a small radii where the box meets the flange, but that's not how the part is drawn.
So lets count and compare to what looks like 60-90 passes around the periphery. (Or more depending on the finish you're after.) Though it's hard to discern from the small photos. Plus I don't see where the holes were drilled or the edge chamfered or broke on the flange top. Engraving? What engraving? Sorry I don't/didn't see it.
3 (or 5 depending) passes down the length of the part using 2 different tools.
10 holes drilled, with 5 retracting deeply into an already existing hole so little time there.
3 times around the periphery with 3 separate tools. (One rough, one finish, one corner round with some 3 axis movements.)
1 time around the flange to machine chamfer the top edge of the flange.
Flip the part over and with a single tool in a pocketing regime, clean out the insides. Nothing pretty, just clear it out.
I would do all of this off a 2D drawing. (Though I may drag out a wire frame to create the corner round path.)