Try Ikon Razors or Bobs Razor Works, both make custom handles to fit gillette and Merkur heads - I'm not sure if the threads are the same on your Muhles.
The iKon stainless steel handles are heavy & the grip is basically slip proof. However it will not match your razors which are chrome plated. If that doesn't bother you go for it. On the other hand you might want to try Bobs as some of his handles are chrome plated. He also has a nice choice of wood handles.
Isn't it nice to have choices
Bob has made me two handles and I have one on a EJ 89 head. He is a very capable craftsman IMO and very reasonably priced.
Check his site posted above. I recommend him highly.