Let's see how this goes.
First, here's a screen shot showing the steps to rounded corners using construction lines (x-lines) to aid layout. From left to right.
1. insert x-lines to define the edges of the rectangle. I show one centered vertically although you wouldn't need it in this case. The x-lines are dragged into place with the Tape Measure tool in reference to other lines. i.e. other x-lines, axes, edges, etc. In GSU I believe you need to hold the Ctrl key while dragging out the x-line. Check the Help files to be sure.
After that, draw the rectangle with the Rectangle tool.
2. For clarity I drew the next set of x-lines in a second step but I would have drawn them at the same time as the first ones if I hadn't been drawing this as a tutorial. In this case, I drew these new x-lines one inch in from the sides. The intersections give me 1" radius corners which are drawn in with the Arc tool.
You could get by with setting a construction point at one inch from a corner along one edge and then using the inferencing engine in SU to guide you for the rest. In this shot I started the Arc tool at a random point on the left edge, then slid onlong the bottom edge until the line turned magenta. If I clicked at that point I could drag out an arc that would make a 1/4 circle.
Back to the first image.
3. I erased the unwanted geometry at the corners.
4. Push/Pull makes the hole.
Now on to the grille.
1. I drew a single diamond element for the grille.
Shown in the lower left corner. This was referenced off the center of the opening with construction geometry.
2. I held the CTRL key down while using the Eraser tool on the vertical and horizontal edges of the diamond. This hides those edges. Then I made that diamond element a component.
3. In the background you can see the nearly completed grille. I copied the component (Ctrl+Move) to make an array that filled the opening. Hopefully you can see the lines from the dimaonds extending beyond the right end of the opening. I've already fixed the left end by making the end diamond components unique (right click), editing them and tracing the edge of the opening on them. Push/Pull eliminates the waste.
Actually I took advantage of a slick Ruby script called applyto.rb to speed things up. There are three diamond components on the edge that need the same editing. I made one of them unique, used the applyto script to get it's definition and then applied that definition to the other two. Then, when I edited one of them, the other two got the same treatment. You can find that script at
http://www.smustard.com
How'd I'd do?