These are all good!
I stopped using a fob with the Coupe because my last ignition switch (after 24 years) was pretty loose due to the additional weight of the fob and house key etc. I know, CF and leather are light but I've learned my lesson. And yes, I'll probably be driving the Z4M 15 years from now too.
Now I use a light duty carabiner. Each key has its own ring. I select the key I need, put the rest in my pocket... and drive. The carabiner clips to a belt loop, a gym-bag strap (inside), inside a fanny pack, etc. No lost keys.
Every year or so I get a new anodized carabiner in a novelty color. Works for me.
My darling bride's X5 starts as long as the "key" is somewhere on my person. I admit I prefer that.
I've looked at folding key shells with thoughts of doing a key transplant but haven't moved on that yet....