Many people building MAME cabinets design and build their own controller boards, and build them into the cabinet. Even if I'd decided to build my own, I would have favored building it as a separate unit so it can be lifted off the cabinet and replaced with other control boards as needed, and so it could be more easily accessed for repairs and such. Just as it ended up with the prefab one I bought from mameroom.net, in fact. But I decided that building that would have taken me a huge amount of time, with all the fiddling with electronics that are pushing the edge of my skill. Plus if there's anywhere I want things precisely machined into perfect alignment and positioning, where I will know it works just right, it's the controls. They're the heart of a MAME system, the real reason you're not just playing on your laptop. So I decided it was worth paying a bit more for a professionally done controller board.
The one I bought has everything made precisely to my specifications, with my choice of colors. I went for a vivid, 80s-brash color scheme that also serves to make it more obvious which controls to use. The yellow joysticks go with their primary buttons in yellow; the orange buttons cluster together for Asteroids, even though they're separated; the white buttons are what control the games instead of being used in the games; etc.
So for this step I was just making the shelves on which it would sit, and be held at the correct angle; and a means of affixing them. The shelves are just two plain pieces of wood cut to size, sanded, and then affixed just like all the others. There's also a notch cut into the bottom edge of the back piece, where the cables feed through.
My initial plan for holding the board in place while still allowing it to be easily lifted off and moved or replaced involved some L-brackets sticking out to the side, and panhead-screws drilled flush into the bottom of the control board which would sit in the holes in the brackets. But it didn't work that well. Making sure the panhead screws weren't so prominent that you couldn't still put the controller on a table meant they didn't hold well in the brackets. I ended up removing all of that.
Instead, I took one of the thin strips of edging that were included in the cheap poster frame I'd purchased, and cut it as long as the width of the cabinet. Carefully bending it, I drilled holes and then set screws to affix it to the bottom edge of the controller board shelf, positioned so the controller would sit snug against it when it was snug against the back shelf.
This was a somewhat provisional Plan B and I wasn't sure how it would work. Would I always feel like the controller board would slip over the edge if nudged too hard and end up falling off? So far it seems to hold pretty securely. If that plastic strip ever feels like it won't hold, I can remove it and replace it with something higher. But for now, the controller board sits securely, and yet can be lifted right up and off. Someday I might build or buy another board for racing games, and then I can just switch between them pretty easily.