OK, so basically my original intention was to have all buttons in red and also DPad ala Nintendo. So I asked here and there and got a set of NDS buttons and also A380 buttons (thanks JodyFox, Anthony)
(top to bottom: NDS, A380, GCW0)
unfortunately the NDS has much smaller Dpad cross so I wasn't able to use it. Also the rubber pads under Dpad and buttons are not cross-compatible with GCW0 without some heavy modification. The size (diameter) of NDS buttons is perfect for GCW0 but they are quite lower (shorter?) than GCW0. But this can be solved as you will see below.
So I decided to go with A380 bcause it looks nice with white Dpad, until I find some better replacement. The only problem is that A380 buttons are something like 1mm diameter bigger than GCW0. Luckily there is enough space to modify the case so the buttons will fit.
Tools you gonna need :-)
So first, you need to make the hole bigger. I've got limited resources so I used drills number 7.5 and 8 (I hope I remember the correct sizes). The GCW0 plastic is quite soft so if you use sharp drills you will need just your hands. Be carefull and work patiently to scrape only the small amounts of plastic so the hole keeps perfectly circular. Use #7.5 and then #8 until the button fits perfectly in the hole.
Here you can see widened button hole
The second thing is that every button has its own outshots to keep the button in the place without rotation. ofcourse they are not GCW0 compatible :-) There is also collar edge which is bigger then is needed. For this you need the small rasper to make the edge smaller to fit in the hole.
Collar egde before and after the modification
The third thing is to take care of the bottom of button.
As you can see, you need to add something there to fill the hole. Also its great occasion to adjust the height of button to your needs. For me the original GCW0 buttons have little bit too "long" step to push the button so I made the fill little bit higher for more "rapid pushes" :-)
you just need to cut a strip of paper (approx. 130mm x 3.5mm), roll it tight and fix it with a drop of glue in the bottom of button
It's in the place finally..
As a last step you need to cut small strips of some kind of sturdy paper to replace removed outshots and glue it carefully in the place. It looks little bit clumsy but I believe its good enough replacement and I didn't notice any troubles with button rotation so far.
So basically that's it. I'm quite satisfied with this mod. Buttons are little bit bigger, closer to each other, they work without any problems or jamming and their surface is acrylic so the fingers slides better between them. Also the ugly "rattle" noise of original buttons is away.