Is there a way to make a K&K battery pack that has the same form factor as the 'Hal' or 'Mal' control modules, so like, they can be stacked up on one another? I'm very not creative when it comes to figuring out good solutions for powering these things as I'm driving new additions to a base into place, and my brain goes, it's silly to add 2x modular storage containers to every new addition.

With the latest update update of KSP to 1.11.0, you can just put some stock battery packs on the surface of the parts and remove them with an Engineer Kerbal once the part is where it should be.
If this is not working, you can take a look at the config files and create one with EC but the model of the Hal or Mal.
This link might help Tutorial:Changing Part Configuration Files