Sorry. I missed all this discussion. How thick is the plastic on the rubbermaid? And do you have smooth surfaces all around the area you want to window (no "steps" like some of the rubbermaids do)?
The process itself is not that difficult and should be easier than it was for the 55. The rubbermaids have fairly smooth transitions and the radius of the curve won't be tight. You'll also be able to install this easily from the inside of the tank, which is what I should have done but couldn't do.
Silicone won't bond to plastic, so the silicone that you use acts as a gasket to seal the mechanical connection between the two pieces. So, you do need a good mechanical connection between the two parts. Since you don't have as tight a radius as I did you might be able to get away with nylon bolts. If you can, I would highly recommend that.
But, figure out where you want it, leave a lot of overlap, drill lots of holes for bolts, install very loosely, shoot tons of silicone in the space between, shoot more silicone in, shoot much more silicone in, tighten the bolts (bolt heads inside the tank is maybe best, maybe not), shoot more silicone around and on top of the bolts inside and out.
Let the silicone cure for a good long time. Then do lots of freshwater testing. _Any_ drips require draining down, siliconing the affected area, letting it cure again, and retesting.
More questions?