Rovner provides the best grip for what is quite a slippery little mouthpiece.
They have a guide on their website that should provide you with a suitable model.

 

Ive always been fond of bergs own ligature. Especially the older ones.

Im refacing a goobered up berg today. My hands are already really tired. They should require a license to use stainless.

 

I use a BG Duo on my metal BL, with a Fiberreed Carbon synthetic reed. It works great but I have to tighten it almost completely so I guess it could be a problem if you use thinner reeds.
/Saxray

 

The Rovner fits fine, but for me it’s like throwing a massive wet blanket on a fire breathing dragon.

I prefer an original Berg ligature. You can order them from their website.

 

The Optimum won't wont work, ok for HR. Metal Bergs come in different body diameters so it can be a pain. I've used a Brancher TMG on some metal Bergs. A "0" chamber Berg may be a slim body and use a Berg metal Alto lig . Try a Selmer 402, should fit.

 

Just to throw something else into the mix (though I'd stick with the original Berg Lig if possible) I like the the Oleg ligature on a metal Berg.
Wilton Felder

 

I have an extensive collection of original Berg ligs & caps I would be willing sell if interested. The BG Jazz ligature is a great lig I use it on my Double ring tenor piece. BG Lacquered Metal Jazz Saxophone Ligature
I use the original Berg lig on my 120 -1 -sms bullet chamber stainless Berg.