Fast Fire III is the one to get. You don't need to disturb the mount on that one to change batteries. Fast Fire II you have to remove it to change batteries.
You can use a Sumtoy mount that's clean and small or you can use a Warne which is similar. With the Warne, you use one base mount of two that come in the pack.
You can google both companies. Sumtoy is on here.
You MAY have to drill and tap one hole if your gun is already drilled and tapped or I think Sumtoy has a provision for using the configuration that's on your gun already.