Maybe an Ergo ar15 gapper plug may work,
Fills the gap between the trigger guard and the pistol grip on the AR/M4 and cushions your knuckle, prevents abrasion to your finger, and provides a more finished look to your rifle. The GAPPER™ is chemical resistant, clips in easily, and secures with a tight friction fit.
www.ergogrips.net
Have been shooting my 1st gen Bravo ,but almost always wear Flight nomex gloves,cut down
Ever since my USMC days,,also they aren't padded like bicycle gloves
I do use a lower grip on my Bravo so never got that bite on my finger
Try rubber cement or silicone .
Or try KRG X-ray rear stock,,