While there will be a lot more dinners to be had from your rod and reel than your rifle over the next few months, I never ignore the Scout's Motto: Always be prepared. Hatchie's back seat gun sling comes in two pieces that fit over and fasten to a vehicle's driver and front passenger seats. Each has a pair of horizontal holsters for storing a pair of long rifles*, plus cargo-type pockets that house shells and additional gear. The sling is made of 3mm Neoprene.
*Amazon reviewers note the Hatchie sling is great for pieces such as a Remington 7400, but might be spaced too wide to keep shorter guns, such as a Marlin 60, secure over bumps and turns.