If you have the piece then go to Lowe's (or whatever you have nearby) and get some Loctite Epoxy Plastic Bonder and use that on it, you'll be fine and it will most likely never break. That's an expensive repair, but considering the difficulty and the risk of breaking the top LCD, it's not the worst price you'll find. However, if you can find, say, a 3DS XL with a damaged lower screen or even a bad motherboard, that's a relatively easy fix (just swap your board into the other system). I'd be happy to do it for you if you can find a system like that.