It's just a foil like material if you haven't figured that out by now. Cut a little deeper but not all the way around, you can use a small amount of adhesive to put the black screw covers back in place, otherwise you'll be staring at screw heads every time you open the 3ds xl.

I would recommend removing the left side hinge, hard to get to it but it does slide out just like the smaller 3ds. Otherwise you will tear the ribbon cables putting them back thru.