I recently purchased Fanatec's button cap/decal pack and I have to say, it's a very worthwhile, inexpensive, and idiot-proof bit of kit. The customization options are incredibly vast, with ~20,000 possible combinations
Customising with Button Caps, Labels and Stickers - YouTube
Fanatec button caps and stickers | Studio-397 Forum
CUSTOM CREATION for FANATEC GT3 Wheel Top Decal – Lovely Stickers