I wouldn't recommend a grip fin. Based on trying them out, I don't find them very practical. Better for range toys you'll only shoot from the bench or something.
Unlike with other firearm designs (like the FAL), the AR's layout permits grip designs that simply alter the angle to meet the letter of the law while providing a much better grip.
On my first build (and soon, on my second), when in CA I have the different grip, a different muzzle device, and a stock stop from Strike Industries installed. Once I cross the border back into free America, I can swap everything out within a few minutes with just a couple of tools.
I would second going featureless on rifles. Fixed mag is the only choice for pistols, unfortunately. It is literally impossible to make an AR pistol featureless.
Cali-legal on top, compared to standard configuration at my preferred shooting spot in AZ. Swapping either way, again, doesn't take long and isn't difficult. If you look at that FAL trigger housing, you can see why such a grip cannot work with that, meaning fin or fixed-mag (I went the latter route; fins are even worse on a FAL, and they can be set up to be loaded using chargers, like in this case).