I'm sure they have all the physical container layout and...

I'm sure they have all the physical container layout and connection permutations figured out.
The objective is to have 100% of the battery offtake going to one power company and they are currently negotiating with two companies.
Iggy wants (and needs) a five year offtake agreement.

Given Iggy stated, 'Offtake can be signed subject to successful battery performance testing', IMO they will wait until the offtake agreement is signed and the price known before announcing/releasing the DFS to the market.

Each Grid pack container will hold 18 battery packs @ 60kwhr = 1.08Mwhrs plus 2 battery sized boxes that house the battery management electrical and electronic systems (BMSs)

So 120 Gridpacks will be produced annually. How frequently they will be delivered and be rolled into the power company's new or existing renewable projects is yet to be revealed. The Altech plant will use several, (maybe the first few), to help power the plant after it's up and running.