Hope my 2 cents worth can be of use to someone here. I also have scoured the web for a large gear bag, and never found anything I was REALLY happy with. What I have settled on is the "ultimaxx" large camera bag. I've only seen them on eBay. The dimensions are:
Inner Dimensions: 19” (W) x 11” (D) x 9” (H).
Outer Dimensions: 23” (W) x 13” (D) x 10.5” (H).
They cost around $50 and I own 4 myself, and have two friends who now have them as well. We are all pretty satisfied with them. The only thing I didn't like about this bag is it has no pockets on the top cover. I solved this by getting a pair of black pencil/pen bags that had two zipper compartments and used Gorilla glue to attach them to the lid. The glue seems to not have any issues being in a hot car for some length of time. I attached them so that I could use the provided pocket on the back side of the zipper case to hold instructions, etc. I have all of my bags MASSIVELY overloaded, and they seem to take the abuse just fine. Has two side pockets and a large front pocket. Side pockets are great for batteries and charger, the front I use for filters, cable release, and even my quick flip for my flash. Adding a better shoulder strap pad if you have a lot of weight is helpful.
Hope this helps someone.