This is easy with strut channel such as Unistrut brand.
You can attach the strut to the casing with strut pipe clamps. You can get these in just about whatever size you'd need.
These clamp the strut perpendicular to the pipe.
With two pieces of strut clamped perpendicular to the casing, spaced the same distance of the mounting holes on the box, you have a very solid structure to attach to. You can then use fender washers inside the channel with nuts and bolts to secure the box to the strut.
That will be about bomb proof. It might seem like overkill, but I agree that even heavy duty hose clamp style clamps don't really do it, and this is quick and easy.
Alternately you may be able to use U-bolts to strap the strut to the pipe, depending on the size of the pipe, but I prefer strut pipe clamps. I think you get a secure attachment without stressing the wall of the pipe as much as the U-bolt does.