With the Rhino flex (at least on mine) if it doesn't fit, turn it 90 degrees and it'll slide right it - I don't know why, but it fits in one way and not the other.
I keep mine in the bumper, but I'm not sure it's the "best" place as I have had some rust staining on my hose, and I wonder if there are long term issues with the bumper. But I hate the thought of having to stick an extra bucket in my storage areas. I'm kind of an organization nut, and hate "extra" stuff lying all over.
2012 Jayco 32TSBH
2004 Ford Excursion
ProPride 3P / Tru Control brake controller
Roadmaster Active Suspension