Wrap some aluminum on the inside as a backer, then solder or have it welded.
If solder just drips off it normally means the metal isn't warm enough, or the metal needs to be cleaner, apply flux (Borax? or similar) to assist adhesion and flow. Use fine sandpaper or bronze wool to make sure it's clean aluminum oxidizes by forming a clear protective coat, the whole theory behing anodizing, so you have to be down to clean metal.
solder done right on clean metal will flow like butter on hot toast.
Ken.