If you took one of these -->
Pelican 1750 Long Case
Add some desiccant gels to keep moisture down , Change those every 6 months just to be safe and your moisture issues will all be at bay.
Pad lock the Case with a pair of Keyed padlocks you keep with your car keys for quick access
And bolt it through the bottom of your trunk of the car into the bottom of a drilled out hole in the bottom of the pelican case , of course you need to use a rubber O ring to keep the case sealed from moisture but i've seen people do this exact same thing before for expensive equipment storage in the backs of SUV's.
The Pelican box then cant be stolen , and cant be opened easily either , is pressure regulated , moisture controlled.
Aside from someone stealing the entire vehicle its pretty much safe from common window smashers , and should not have problems either with the elements.
Vehicles are becoming tougher and tougher to hotwire now a days so thats basically the only other last concern with leaving a rifle in the trunk.
But its a 160$ sks and although some dirtbag getting his hands on something like that , if you report it stolen with the vehicle then its not at least going to be against you.
As far as legalities i doubt any cop is going to say a damn thing about a firearm secured in this way and unless they are searching your vehicle for unrelated purposes to begin with they would never know about it anyways .
also as far as ammo , Keep a few mags or full strippers depending on what you use, inserted in the foam and your good to go
PSS: I was also just thinking if you wanted it to double as a portable case you could remove easily. You could secure the bolt used to bolt the pelican case down with a nut outside the case underneath it so it doesn't move if you unbolt the pelican case. and then on the bolt that enters into the pelican case go [ oring > washer >thick cotter pin ] obviously through a predrilled hole in the bolt, and just leave a cutout in the foam all the way down to the bolt/cotter pin.
and if you ever want to remove it pull the cotter pin and wala you can pull out the pelican case from the trunk in minutes. It would still be secure since it needs to be opened in order to be removed but also removed easily while maintaining security to vehicle.