I should see them work like nestable boats with a limit to the number one can stack in a combine window like wagons.
Incidentally I should like to see boats carryable by cart or wagon. with obvious nesting limitations. If the boat would take four spaces that'd give for being able ot put chests in the boat but restrict, obviously boat in a boat but and this suggestion are probably just something for loftar to mull over while considering the revamp on the inventory system.
in the end it would make creates take more space as you would need to restack them. Perhaps empty crates might stack higher to allow some creative designs and emergent gameplay.