Nice mod, although I’d love to see an alternative mod that adds a cargo wagon mk2, with a more expensive recipe, locked behind a technology research requiring purple (production) science packs. For that mod, I think that it should only have double the slots (so, 80).

That is an excellent suggestion for an alternative mod, and also out of scope for this particular mod. Currently this mod only modifies existing prototypes and while adding new prototypes would be a lot of fun it would also be a lot of new code.

Also, the current value (which is dynamically calculated based on the inventory size of the steel chest) was chosen for a specific reason (it means a normal train station buffer with six chests per cargo wagon can hold exactly two trains worth of cargo, which is the same ratio as a normal fluid buffer with two tanks per fluid wagon).

If I were to implement something like that, it would probably be three tiers of wagons with (assuming vanilla chest sizes) 48, 96, and 144 slots per wagon respectively (lining up with buffers of wood, iron, and steel chests). I may indeed do that in a separate mod someday, but I can't make any guarantees about when or if I'll get around to it.