One of the few good applications of a shoulder rig in my opinion. Fobus makes one that is virtually indistructable. And actually quite comfortable.

 

maybe a plastic gun case that fits in that space. i know my sig came with a gun case of that kind. also the leather handgun cases or the mesh/textile case could be weather proof. that might fit under the seat. never tried it. when i go shooting i bring the truck. if you really want it to be weather proof take a gallon ziplock bag. take the case. throw the gun in the bag. then put it under the seat. best form of weatherproofing i can think of

 

You could use the foam out of a pelican case to sandwich the pistol between the undertail and rear seat.