Dimensions inside the rectangle might be an acceptable option, you could add a caterpillar on the edge of the rectangular object to simulate a partial hatch.
Another option, you can always make the dimensions and then explode them, and shape them as you see fit. Obviously this renders the...
The easiest method is as stated above, find the major part with that hole pattern in the assembly on the drawing modelview tree, right-click and show hidden lines, then hide the lines that you don't want.
