For this idea, I use a Maxpedition Jumbo Versapack multi-purpose bag for a camera bag. Maxpedition gear is made very well, and designed to hold up to tough use. The bag is double-stitched throughout, and uses high quality YKK zippers.
The Jumbo is available in several versions and colors, including Black, OD Green, Khaki, Dark Blue, Royal Blue, Yellow, Red, Orange, Maroon, and even Pink. The various versions of the bag include:
For this project, I am using the standard - original Jumbo version.
I would recommend either the standard or "S" Jumbo or EDC versions. The EDC version's side pouch can hold most speedlights, while the standard Jumbo cannot. However, the rain sleeve for the main compartment is removed on the EDC version. But this is not a problem as I will explain later.