Mopar Slush Mat
I have a front and rear set of Mopar (Agate) Slush Mats in my Ram along with the factory "rear underseat storage" (in fact, I waited until this option became available in 1998 before ordering my '98. 5 QC 4x4).
The rear, one piece mat works much better than the Mopar carpet mat I purchased at the same time I purchased my truck. BTW, the rear Slush Mat would even work if you had the large, Mopar locking underseat storage compartment.