So I took some pics of the inside of my keyboard, hope it helps:
http://imgur.com/CnsrQsd,ceS8rTP,LGndfAD,Zdq51UI#0http://imgur.com/CnsrQsd,ceS8rTP,LGndfAD,Zdq51UI#1http://imgur.com/CnsrQsd,ceS8rTP,LGndfAD,Zdq51UI#2http://imgur.com/CnsrQsd,ceS8rTP,LGndfAD,Zdq51UI#3Like Butterfree said, I cut out the numpad area for the PCB. I didn't even use plexi, it's just a piece of plastic.
For simultaneous left+right (or up+down), the PS2 pad PCB I used registers as neutral.
Figuring out how to mount the tiny switches was the trickiest part. After a couple years of use,
I ended up just hot-gluing them, because a few of the solder points would break after months of use.
Since I removed all the rubber pads, the keyboard keys directly hit the micro-switches, with some padding.
I got used to the feeling of it, but anyone else might want to keep the rubber pads, or get different switches.
This is the micro-switch I used:
http://is.gd/S3Xuza, it seems to be pretty durable despite the size.
Also all the wires are hidden underneath the plastic sheet with holes that came with the keyboard.
So I'd rather not try selling another keyboard like this, because mine is pretty much a hack job.
It took me two revisions and a few days combined to get right. And also I needed to repair a few of the solder points several times.
Sorry for the wall text, I kind of just brain-dumped.
Hopefully this helps people who want to do something similar, and I can always give advice.