I whipped up a clone of that breakout board in Kicad. If anyone has the exact dimensions of the board that'd be helpful. Otherwise It can jsut be As Is. I will whip up a 3D Model of the blanking plate this week some time that will fit this design. I haven't built or tested it yet as I don't yet have a P5A-B (although I am on the lookout).

If anyone wants to sanity-check it, please let know if you find any issues. Also, I'll upload the gerbers for it if someone wants to have a go at building it. It's all 0805 SMDs and 2 electrolytic capacitors, and the USB and PS2/IR connectors are standard.

I don't know if there are really any critical dimensions other than you can fit it on a bracket. Personally, I would size it so as many as possible fit on the "$2" PCB size, what's that? 100x100mm, so you could get maybe 3 on it with perforations drilled to separate them. Also, I would try to get maximum USB port spacing for fat dongles and thumbdrives. Though maybe PS/2 spacing needs to be preserved for use of adapters. (AT-PS/2, USB-PS/2 etc)

Another thought would be to make it just narrow enough to fit in a 3.5" drivebay, so you can use them on a floppy blanking plate maybe.

Yeah it could easily work with both a rear-facing plate and a 3.5" Bay. The final dimensions are 90mm x 41mm. Gives plenty of room between the connectors, (more than the original), and still fits comfortably in a rear-plate and a 3.5" Bay. Two mounting screw holes are 2.5mm so will handle clips or some M2.5 screws as well. Updated the labelling, and tidied up the layout.

Will put the source files up on github for anyone that wants them, including the gerbers.

https://github.com/optoisolated/ASUS_P5A-B_USB_MIR_Breakout