After showing the 3 hole mounting version for my printer to a couple of friends, a few of them mentioned that they would love a 4 hole mounting option, so here it is.
3mm Aluminium should do well for your printing surface on top of your PCB MK2a Heatbed. It's 214mm x 214mm with cut-outs for the set screws and electrical connections on a Reprap PCB Heatbed MK2a which will be mounted with 4 posts.
3 hole version here: http://www.thingiverse.com/thing:236148 but I think this version should work on a 3 hole mounted bed as well.
Update: Version 2 - After having these cut, all seems to fit well and really tight except that I made a mistake with the cut-out for the electronics. Version 2 the gap is really 80mm wide instead of 70mm in the original. If you use your heatbed with the copper side down, the original may still work fine which is why I left the .dxf file.
I added a tab about 15mm x 5mm sticking out near one of the corners to allow for it to be easily lifted off because this plate is exactly the same size as the heatbed.
Minor update: Sharp corners in the electronics cut out.
Easy bubble free Kapton tape: http://youtu.be/HahEdDYf7wQ