Weld-on is the necessary glue if it is acrylic on acrylic to bond, if you care about looks. You need to go to acrylic supply store to get weld-on. There is a thread here listing that info. Epoxy plumbing repair putty (like Aquastick for frags and live rock) is available at any home center. If you clean both parts, rough with sandpaper, then clean again to get rid of dust residue, you can fix anything with a wad of that stuff, if you know what you are doing and wet your hands while doing it you can tool it to a reasonable finish. Good luck! If the top is polyethylene plastic of any kind, it may be hard to adhere securely with any glue, including weld-on. If you have to make a fillet of epoxy around the joint to hold it that may be a good, strong, permanent repair.