I use the outdoorsman's arca rail for my spotter.
Why not use RRS ARCA rails?
If you’re just needing a plate between your Anvil 30 and your panning head (which is what quite a few people do), then about any Arca plate will work. The clamping force and width range of the Anvil 30 can accommodate fairly large variances in plate widths.I have a RRS with an anvil on it
I plan on putting a a panner in it to make it easier to glass.