Madhouse Design has a billet pistol mount that holds your pistol with a rod. It looks like something a dealer would have to display pistols at a trade show. You can mount this under your dash or under a desk. It retails for only $49 on their website.
There are rods of different thicknesses for different calibers.
Ok, so let’s critically analyze this pistol mount. It requires the pistol to rest its weight on a rod inserted into the barrel. Nothing terribly wrong with that. I do have issue that there is nothing retaining this pistol other than gravity and possibly friction depending on the diameter of the rod. The other major issue is that the trigger is not protected. If you were to have this in your car and you were in an accident with a possible roll over, the gun could come loose and the trigger would not be covered. Posing a risk that is not needed.
You could solve this little dilemma with a simple kydex trigger guard cover or something like the Raven Concealment Vanguard tethered to the mount. That way the trigger is protected and the gun would be tethered. Yet you could still pull the gun with enough force to over come the kydex cover and draw the weapon.