Help - fixing to C-pillars

[cubcampgrumpy]

I’m looking to attach cup holders to the C pillars for my kids. They’re carpeted and appear to be metallic underneath. What’s the best way to fix to these? I was going to drill small pilot holes and use short self-tappers. Is there anything more suitable? Thanks

 

[TeeCeeJay]

Rivnuts? They are metal under that.

 

[Howzat]

Magnetic Cup Holder
I use one of these on my bulkhead but thats magnet to bare metal which is really strong but not sure how well it would work with carpet.