My idea is connected with the way in which trunks work. In my opinion, putting containers (like a backpack, duffel bag, military rucksack etc) into various trunks should increase their volume. Consequently:
An ordinary car trunk: 162,50 L of volume
A duffel bag: 30.00 L of storage and 6.00 L of volume
If a player put a duffel bag into a trunk, trunk’s volume is reduced to 156,50 L. Ok. But what when this player will put a SIG 552 into the same trunk? SIG’s volume is 2.00 L so trunk’s volume will be reduced to 154,50 L. Is this duffel bag filled with air? Player should be able to put various things into containers that are placed into a trunk.