It could also be made from 2 straightlines and 1 arced line attached and welded s necessary then extruded (but martinvks method is easier). There are almost always at least 3 ways to do anything in Gmax.
Heres a 3rd way: Make a cylinder the correct height and diameter then make two rectangles larger then it and use them to boolean transform away the unneeded parts (but again martinvks method is better).