I don't have that big a window, but I also have a black foamcore light shield. In my case, the 32x40 full size foamcore board wouldn't lap over the molding outside the window, so I had to get a little creative. First, I cut strips of black-core matt board to make a frame insert that slips into the window frame; this is assembled and attached to the foam core with black masking tape. Then I used more strips of the same matt board, scored and bent, to extend the edges of the foamcore and lap back over the window molding on the sides and top (since I can't paint the window molding or frame black, I need a little extra help here to tame reflections). At the bottom, where the sill prevents lapping over, I used a double gasket, each layer made of two strips of the black tape, sticky sides together, one at the glass-side edge of the inner frame and the other at the outer surface. After a few uses, I had trouble with the cover popping loose at a couple corners, and found it had developed a mild warp; adding similar gaskets on the three other inner frame edges both added friction and further cut down reflections.
My bathroom/darkroom is now dark enough (providing I remember to leave the fluorescent over-sink light out for a half hour beforehand) to handle film at EI 4000 or higher with full sunshine outdoors (though it surely helps that the window is on the north side of the house). I can go from bathroom to film changing (i.e. no chemical mixing or pulling up the rugs to get the enlarger cart in) in about five minutes.
Cost to make the unit was about $12 -- $10 for the foam core board, and a proportion of the $7 matt board and $8 black tape (for which I've found dozens of uses since thinking the roll was outrageously expensive).
Your huge window is going to be difficult. If it's a multi-sash, you're home free -- make multiple foam core covers like mine, and they can butt against each other at the center frame(s), assuming the individual sashes are narrow and/or low enough for a single sheet to cover. Otherwise, I'd suggest looking at Masonite, aka pressboard, as an option for the main cover; you can get this in 4x8 foot sheets, so you can cut to lap past the frames all around in one piece, if needed. Bad news is, that big a piece of Masonite will be heavy -- like 20-25 pounds -- and if it's all one huge pane, you won't find it easy to make a multi-panel cover that will be light tight at the joints.
