More gas tank sizes (no new models needed!)
The issue is that the hydrogen tanks are too huge and sometimes oxygen tanks are too small.
The current O2 and H2 tank models can be used as alternate tanks for the other gas.
This could be implemented in a simple way by adding 2 H2 tanks with the O2 tanks and vice versa (4 total)... and maybe a small edit in the models to change the writing on the side (either to a specific gas or just generic "Gas Tank").
Or it could be implemented in a nicer way: the current tanks turned into generic tanks that can hold either H2 or O2 (or any other mod-added gases/liquids, a resource type would be needed in the definition), that choice being picked in terminal and would require venting of existing resource before changing it.