How about this then:
*Truck Bed = 1000 (Insecure, items have a chance to fall out/shift unless you bound it with obstacle tiles, also means you need to close your tailgate!)
Trunk = 600 (I really think 700 could be argued as completely realistic, but I feel 600 is a “balanced” value)
Floor Trunk = 400 (I shouldn’t be too much less than a trunk, as the benefit is rather minimal it’s not even 4 cu ft)
Bed = 200 (assume stowed below sleeping surface)
Seat = 100 (assume stored under/behind)
Aisle = 50
Box = 300
*Bike Basket = 100 (found on bikes, 0.8 cu ft)
*Stow-Board = 300
Mini-Fridge = 300 (2.6cu ft, I think this is spot on)
RV Kitchen = 300 (One standard drawer plus a small cupboard could reasonably hold 2.6cu ft)
Weld-Rig = 200
FoodCo Buddy = 100 (One drawer, 19x4x24in is ~30l so 120vol would be equiv)
Forge = 50 (as per the PR pending)
- Items do not yet exist in-game
I think most of these values are grounded in reality (and if anything are undersized, potentially reflecting non-careful packing), and represent reasonably balanced cargo capacity.
Edit: Changed mounted forge to 50vol to reflect the PR pending on it.
I am playing now with these values to get a feel for how well they work (sans bike baskets and truck beds).