Thanks Alga!
I was just going to have steel strips with foam on the bottom, but I made such a mess cutting the strips I came up with the idea of using the trim to hide the ragged edges
David, my straps don't follow the lines of the tank perfectly either (in particularly down the rear!). They are tight against the corners of the tank though, so should hold it in place.
I did take a photo of the rear (it's in my photo album), but it's not very clear.
I'm in the middle of fitting the ali tub at the moment, so can't take another right now. It'll be coming off prior to final fitting, so I'll try to remember to get one then.