I got my hands on a free promotional backpack and needed camera storage, so this happened. Kaizen foam used to organize parts, grommets and paracord used to hold the large tripod on the outside of my bag.
I start with the freebie backpack and cut the gasket things that keep the flap from opening all the way because, well, I want it to open all the way.
The backpack is turned inside out so I'm able to get the exact shape of the inside of the bag.
I use a piece of cardboard and trace the backpack on it. Then I cut it out to use as a template for the foam that will fit in the backpack.