The drawer was almost an afterthought for me. It was just an idea that maybe it'd be a good idea to have a place for a keyboard and mouse for the times I have to access Windows stuff. While my computer came with wireless ones, I thought it might be nice to use a wired one, and save those for something else. Then I thought, plus, I can store an Atari controller in there and load the 2600 emulator. How hard can it be to throw a drawer in?
Well, it turned out to be the hardest part of the plan, and if I had it to do over, I would either have done it very differently, or not at all. Making a drawer is not that easy, and making one that comes as close as possible to disappearing entirely is even harder. I would probably have instead made a hinged door with a sliding drawer inside it, or something. If you have enough skill to make a drawer with a flush front, like the kind on kitchen cupboards, that would be ideal, but I don't know how it's done. So I'll document what I did, but keep in mind that the result doesn't look too great, despite being hidden by touch-up paint.
First, cut a front plate for the cabinet, with the top edge beveled at the same angle as the controller board. The plate should be shorter by the thickness of the boards you're using, so that when the controller shelf sits on top of it, it'll fit against that top edge.
You will want to build the drawer a few inches less wide than the front piece you just cut, and you'll need to put a few strips of wood inside to mount it on. Based on the thickness of those pieces of wood, figure out the dimensions of the drawer and cut the pieces to suit -- using the table saw! -- so that the front panel is all one piece. Jigsaw out a finger-pull in the front, and a notch for cables in the back. Sand all the edges as smooth as you can.
Next, you'll need to trace the front of the drawer onto the front panel of the case, and then jigsaw-cut out a hole to match. The ideal would be for it to be the same size and shape, so that the drawer is flush. But that's not possible. First, you need gaps for the drawer slides on the bottom on either side. Second, you need a gap on the top if you want to be able to lift the drawer out. This is where mounting the whole thing with a two-layer front like on kitchen cabinets would help.
Now you can mount the front plate, as you did the base, though you won't need corner clamps anymore.
Then follow the directions on the drawer slides to mount them. Expect to do a lot of adjusting, trimming, sanding, and rejiggering things to get them to slide evenly. Later you'll need to paint all the edges with black paint to hide the joins as much as possible.