One of the few grips seen with the tanodized CAG HK416D in the wild is this one by Sierra Precision.
It’s a fatter grip, which is said to fit the palm better than a standard AR grip. It’s also the same angle as the standard AR grip – unlike the issued HK V4 grip.
Here’s a couple of reference blasters which feature the Sierra Precision grip – from the chopper pics.
While the issued RAL8000 HK V4 grip is easily the most popular choice in the wild, judging by pics in the public domain the Sierra Precision grip is a verifiable alternative; and may be more suitable for those who prefer a shallower angle.