This is my first attempt at a MAME cabinet, so I figured I'd start with a bartop. I wanted something that is a little "sleeker" than the boxy type styles I commonly see, so this one is modeled after an old computer design.
Originally I was going to do one that looked like an old commodore PET, but I wound up doing one that looks more like an old TRS-80 Model 3 than anything else.
I got the monitor sized up and just "drew around it" to get the most compact design. The best way I could figure not messing up the clearance was to use an actual picture of the monitor I am using (an old GoldStar 14" CRT I had lying around). It's design is actually very similar to the 'PixelBox' project on here, it's just much bigger.
I took a side shot, brought it in to PhotoShop, scaled it correctly, then verified all the measurements again to make sure. Following is the profile I came up with:

I won't be using t-molding, I am building it all to "seam up". The hinges will allow the whole top of the cabinet to fold down, allowing easy access to everything. It's 24" tall and 26" wide. Plywood for the base, and 3/8th MDF for the other panels. The monitor rests on two wooden dowells (the notch on the bottom made it perfect for that) and there are a few 1x1's and 4x1's used for bracing and support.
I had a bunch of computer parts laying around, so I wound up putting this together: