Placement of a lock/mount depends on the particular weapon system that you have. In a crown vic I have found that a 20" model doesn't fit real well vertically between the seats, the muzzle scrapes the headliner getting it in and out. For a 16" or smaller this is not an issue. You will find that if it doesn't fit vertically then it won't fit well horizontally either.
What I ended up doing is this: I rebarreled our 20" A1's into a Tacical-16 or what some refer to as a dissipator configuration. Our weapons all have surefire millenium lites on them as well, which is also an issue with the lock head. The standard lockheads that I have found from Santina, ESMET and Bigsky will not fit around the millenium foreend due to the pressure fence rail. In our cars we carry the carbine as well as a Benelli entry gun. We decided to have both of them mounted vertically between the seats. I ended up making the bracket and obtaining a very small lockhead from ESMET that goes around the barrel. I made the bracket in the traditional "L" shape of most vertical shotgun lock brackets. I used a shotgun buttplate.
I then took the small lock head and mounted it so it's position would be around the barrel in front of the front site base. I have made several of these for neighboring agencies as well.
As far as the hot/cold issue. Keep the gun wiped down on a regular basis with a good lubricant and protectant. I highly recommend Militec-1 for this. It has greatly performed in subfreezing, snow, rain, and heat for our agency. In December I taught a rifle instructor course in Tennessee. During the armoring portion I had everyone lube with militec-1 on everything since we had the rifles completely disassembled. We had nine new rifles of 14" M4 configuration, and a couple of 11" commando's in the class. We shot 3 days in cold rain, even prone in the mud, day and night with an average of 1000 rounds through each rifle. Six of the M4's were fed S&B, and others were fed Ultramax. All rifles functioned flawlessly, which is rare for lots of brand new short barreled rifles with dirty types of ammo and wet dirty range conditions. I have found no better corrosion inhibitor and lube than Militec-1. I would also recommend rotating your carry ammo at least once if not twice a year as well.
Just my $.02
Sully
DPMS Master Instructor
www.defensive-edge.net
www.dpmsinc.com