Weld? Bit of threaded rod or a bolt with the head hacksawed off. The ID of the tube is about 4mm so M4 rod / bolt should do.If it’s a bit loose, some fine wire or solder wrapped around will do. Too tight, ream it with a drill or file. You can always use a few spots of superglue to hold everything in place as there will be plenty of metal to metal contact.
In fact you’ll probably not even notice if the reflector is a bit short. Sure, maybe the gain or pattern is a fraction of a dB off but so what.