Uncle mikes trunk organizer is your best bet - Uncle Mike's Trunk Organizer

Very well built, very well designed piece of equipment. Large enough for a complete AR carbine, plus other gear.

Excellent compartmentalization options, with quick-adjusting hook/loop securing of partitions. No personal experience setting up bag with heavy ammunition or liquid spills, but bag is certainly durable enough for both. Non-skid lining in bottom.
Organizer has great tie-down rings sewn into bottom, but nylon webbing is not provided. Top panel has nice tie-down ring to secure half- or full-open lid while using bag in your trunk.

Strongly recommend measuring trunk of the intended vehicle, and using measurements for bag. Any trunk roughly 16 cubic feet in volume should be usable, but I have not measured more than one car.

Large lid with great MOLLE loops provides ample attachment points for lights, pouches, etc. clip-on lights or flashlight with clip is suggested for easy use, as this bag becomes a 'black hole' when stored in your trunk. It is nearly impossible to see the organizer resting in an open trunk from a distance, unless bright gear hangs off of it or sits inside it (road cones, reflectors, etc.)
Pros: Durability, construction, compartments, MOLLE panels, tie-down points, ease-of-use, handles
Cons: Size makes it difficult to maneuver in some trunks, and may require removal for long-haul trips with baggage in trunk.

This review was written in the old system and had content requirements that are different than reviews written today.