You can get L-brackets in all sizes at hardware stores. They're made for holding shelves, bracing inside corners, and a whole bunch or other stuff if you get creative. Making your own is not advisable because they may bend, but they're cheap and sturdy. They come in sizes from about 3" all the way up to around 1' depending on what you need them for.
For your project I wouldn't advise anything less than 4", but 6" is probably better. Using 2 would be good, but 3 would be better.
On top of that, you'll going to need some method of attaching the whole thing to the cabinet. You can't do that with just corner pieces or L-brackets alone. Those pieces would only prevent the whole thing from falling forward. It's still gonna want to shift all over the place. You're going to want to attatch it at the sides somehow to prevent lateral motion because those L-brackets will support a LOT of weight vertically, but they actually have the ability to twist a lil bit. You'll understand this more when you see one. Just grab it in your hands and try to wring it like a towel. (It's more noticeable with the larger sizes.)
A control panel the size of what you're thinking of could easliy weigh 30 pounds or more depending on the materials you use. (Good question. What materials are you using?) You have to think of the front edge of your cabinet where you want to attatch the brackets as a pivot point and the more weight you have over hanging in any direction the more it's gonna pull your whole CP down around that pivot point. Depending on what method of support you use, that whole thing may pop off and you'll be playing indian style on the floor.
OK... enough about that already. I've been writing this post for like 10 minutes.