If you want a button that isn't nearly as expensive as the z8 one, just go with a s2000 button. You can get them new online for ~$35.00. I put on in my car and it's still one of my favorite interior mods. There was a DIY somewhere out the once upon a time that I used, but I couldn't find it.
And as far as being inconvenient, I figure I'm gonna more my hand down to the shifter anyway, so I can just press it then. It's not like it's placed under my seat or anything.