It is looking like the R1T will be sold with different box modules according to some new patents that have been filed. This kind of versatility is the exact way Rivian can distinguish itself in this segment and I hope these become a build option from factory. Looks like another forum has done some renderings of what may be in store.
https://www.carscoops.com/2019/03/rivian-patents-intriguing-modular-box-systems-for-the-r1t-pickup/

https://www.carscoops.com/2019/03/rivian-patents-intriguing-modular-box-systems-for-the-r1t-pickup/