I modified one of the cupholders in my 2002 Boxster about 2 years ago.
I have been using my fairly heavy SS coffee mug with this mod all this time and haven't had any issues.
So, here is how I did it:
1) Removed cupholder
2) Located center on the bottom
3) Used a holesaw to cut a hole slightly smaller than the top hole
4) Cleaned up the edge of newly cut hole
5) Reinstalled cupholder
