I'm not worried about leaving my firearm in the car when I leave; I home my situational awareness is better than that. The holster is for holding while driving, not to have as a 'truck gun'.
Just had another thought, brought on by something sold at a Very inflated price.....
Depending on the size of ya gat....
A) Obtain an appropriately sized cup from the local stop'n'rob.
B) Find some Acraglass or perhaps quick-set latex(?) from the crafts store
* thinking on it, I'd recommend something with a bit of 'give' in case you miscalculate how much is required, latex is sounding better and better.
C) Wrap one or two layers of stretchwrap over the front if the firearm and spray it with a release agent.
D) Fill bottom of cup with your concoction to provide a base and stick the wrapped muzzle into the cup.
E) Allow to dry and you have a custom shaped, low profile holder for your choice of carry.
Your avarage quick-sip cup might be a bit flimsy, no one says you can't use something more substantial. Heck, if you had the confidence you could mould the carrier directly into the cup holder, so long as you lined it and used quick-release agent everywhere.
(I knew a custom knife maker back in the 70's who used to have a gunrack in the back window of his Cadillac. I'm drawing up plans for a horizontal carry rack for the back of my wagon)