I literally just did this 2 weeks ago with a few differences. I did NOT run a negative wire all the back and instead grounded directly to the chassis because the battery is connected directly to the chassis also. I'm still having some second thoughts about that option versus a dedicated wire.
How are you going to fabricate the brackets, especially the one up front? 3D-Print or metal?
If you don't want to drill any holes, you can get at least one 1/0 gauge wire and possibly two through the grommet in the rear quarter panel behind and below the 12V battery. See below where the grommet is (battery, battery tray, and jack mount removed). I ordered some of those waterproof glands for 1/0 gauge wire and they are massive. You have to drill a hole that is something like 1.25" in diameter.
