I've never seen replacement housing for 3DS XL in person, but I've seen lots of replacement housing for the original 3DS and none of it looks or feels like the original. I get a fair number of consoles from a repair company that, for some reason I cannot understand, always replaces broken housing with aftermarket housing and it's always obvious in person that the housing has been replaced. Here's a comparison showing the aqua blue housing for the original 3DS, just so you can see how bad it is (aftermarket housing is on the left):
It may look exactly like the original housing in photographs on eBay or Ali, but in person it's completely different. Blue is the worst, but even the black replacement housing is not quite the same as the original. The feel of the plastic is a bit different, the font of the ABXY buttons differs slightly from the original, and there is even a typo on the back battery cover present on all aftermarket housing I've seen for the 3DS.
Now, maybe it's possible the replacement housing available for the XL is better than the housing I've seen for the original 3DS, but I wouldn't bet on it. Go with original Nintendo housing that people have salvaged from broken consoles. It may be more expensive, but it will look original.