Best glue. I do not think there is one. Ebonite is a rubber based material. So my guess would be an epoxy but not one of those quick hardening epoxies. I would choose something like System III T88 or if you want instant glue Loctite 480 ( but it is black in color)
If you are gluing ebonite to ebonite it would probably be beneficial if you used a tenon system if possible to give more stability to the glue joint. Good luck.