Looks like they got ya pretty squared away really. What I'd like to add to all this, is something that was not mentioned, or not made clear. You can cut the sheetmetal out in any bizaar fashion, and still use one square piece of Lexan or glass. If you're thinking that cutting the lexan out to match is going to be a pain, then forget that and think a little different here...
Before you even start chopping away, you can get a piece of Lexan and cut it to fit the inside of the panel you intend to mold into your design. Once it's all cut out and you call it good, you can then apply Liquid Nails around the cutouts (on the backside of course) and then put the Lexan sheet on. You may want to squeeze it down pretty tight, and wipe away any that may seep out and become viewable. Once it's good and dry, you won't be able to tell if it's custom cut or not...and it really doesn't matter because it was way easier than trying to cut each piece to fit. And after it's all done, it'll help getting the panel more rigid so you can actually open the thing without worrying if a panel will fall out.
My opinion on cutting has pretty much been said by these cool dudes already. First, it's all in the design layout. You can do a great job and all that, but the design could still just ruin the entire look. Grump got ya a lot better, because I can get into this and start sounding like the Queer Eye for the Geek Guy:p So, picture this minus me with a limp wrist...Deal?
I think you have a good idea with a basic design, and not going with some branding label like the AMD symbol or stuff like that. Besides being un-original, this is a palette to get your art out there and on your case; so do it. That design is blocky, but THAT'S MY OPINION! I like roundy, some like blocky...big deal. It's still yours, I'm just tossing this out there. Roundies make the job a bit lengthy for some.
Grump pointed out some areas that you may not want to be seen. I'd say that the drive bay area, the part up top where the wires come out of the PS, and maybe even the bottom of the case should be left out from being open to the public. Unless you have something interesting down on the base, I'd leave it as is to hide some wiring, the boring NICs that can be in lower slots, and to hide a CCT if you use one. I'd show off mainly the pretty parts, and that's the guts of the board. These heatsinks have always been kinda pretty to me, and the circuitry of the board is neat to look at. Accent those and make 'em stand out. Just more opinion
To make a roundy corner, you can use a template. A portion of an aerosol can cap, a gatoraid lid, a skoal can, a CD...whatever makes that corner.
Just ensure the apex of the curve leads out to the straight edge. This can be done by placing the sharpie on the line you've already done and angling it away from the inside of the area intended to be cut out, then butting the guide (like the gatoraid cap) up to it. See, if you just line up the cap to the line, the thickness of the pen's felt tip can actually draw outside of the real area, because the Cap took up the intended line you are to cut and not the felt...get it? Just trying to point out some basic drawing stuff that may help...may not. You get something like this:
and not this:
To get it right, you need a decent eye. But a first, it really doesn't matter...just don't chop your fingers off or get hurt. You can be sloppy here, just get close to the line without going over it. The closer, the better because it's less detail work you'll have to do.
I'd work on the flats first as stated in a prior post. A sanding block with some rough paper does wonders. just sand it down until you get on INTO that sharpie line. It's good to remember how you drew this out here. If you use a ruler (straightline), then think about where the ruler was when you drew it out. The part that was on the ruler, was the straightest guide you drew, while the back side of that line varied with the amount of pressure you applied to the pen. Some more basics there. I have the ruler on the outside of the cutout, so the outer most part of the black line is the guide, and the inside isn't something to follow...and that's why I say sand on into the black line some. When you finish it off, you'll sand on even closer right up to the line until you can't see it anymore. If you follow a curvy line, you'll get a curvy line and it'll be noticed.
The result should be this:
Why a block again? Your fingers are rounded, and your hand is uneven. Think of this: when you sand a panel's surface down with just your hand and sandpaper, your fingers and palm are applying the pressure. So, if you sand this way, it's not flat because you actually sanded grooves in. A block won't do this:
The edge is the same really. The block is a flat guide to help ya along getting that straight line. It's flat, so the line should be flat as long as you do it all in uniform and with even pressure. To get those sharp corners sharp, bend the sandpaper around something squared and get in there.
That's really really basic, but I hope it did something good.