Hey, Adam!
I wouldn't worry about it too much ... you can either leave it just that little bit wider all the way 'round and let the roof edge moldings cover the gaps at the edges (
some aluminum stock is 49" wide, and if you find some, that'll be even better) or you can "pinch" it in to the 48" overall width at the top and then let it "flare out" to the 48-1/4"-or-so on the bottoms (you can even adjust for that "flare out" when framing up your galley lid and make sure the overhang matches all the way up to the galley lid hinge). IMHO, little "adjustments" like that are no big deal ...

...! Good Luck, and as always ...
CHEERS!
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