With the drawer done, things will get a little easier. Actually, each step will probably be easier than the previous one from here on out. Which also means each step will still include one tricky bit.
Cutting the speaker plate is easy enough. What's trickier is mounting the speakers. If you are using cheap PC speakers like I did, you'll want ones whose face is flat, but you will have to figure out your own trick for getting them mounted. I'll show how mine worked just because it might be illustrative.
The plastic cases for the speakers I used were held together at the top by a pair of screws. At the bottom, there was a hollow space on which they stood, into which the cables connected. These proved the methods for affixing them.
First, I traced them, then drilled and jigsawed to cut out holes for them. I made the holes too small, then spent an annoyingly long time trimming just a bit more, sanding, trying to make it fit, and repeating. This was necessary to make sure they fit as snugly as possible.
Once they were in place, I turned my drill as far on its side as possible and drilled through the hollow parts at the "bottoms" of the speakers (now, the faces turned towards the plate's center) into the wood. Some short panhead screws went in to hold the speakers in at that end.
The fit was so snug, I might have stopped there, but I didn't. I cut thin strips of wood, only a few millimeters wide, and longer than the speakers are wide. I removed the screws that held the tops of the speakers together, then found longer screws that would also fit, and drilled through the strips of wood to set those screws in place. The result was that each speaker had a brace across the top. I could then drill and screw it into the wood to secure the top end of each speaker in place.
The grilles I used are a little more grey and less black than I would have found ideal; it'd be perfect if the speaker grilles almost, but not quite, blended into the wood, so you didn't notice them at first, but when you did, they looked like speakers. But it's far more important that they be flush. And when you look at it, it looks like it's supposed to be just how it is. (In this picture, there's a bit more visible white wood edge, but that's covered up with touch-up paint later.)
The speaker plate is also the bottom edge of where the marquee will go, so I had to cut and route a groove for it here just as I did on the top, though at an angle this time. The process was pretty much the same despite the angle.
Once the plate is done, I installed it as I had the front. My design also has a 2" high "bevel plate" in between the speaker plate and monitor, which I also cut and installed at this time. That bevel plate serves no actual purpose; it just gives the front a little more sense of contour.