Really liked Ultrasonic2's "Nerf Stryfe Front Grip" (https://www.thingiverse.com/thing:2400385) which was sized for Picatinny Rails.
Took the Nerf Tactical Rail attachment from linka71's "Nerf Front Grip" (https://www.thingiverse.com/thing:431863) and combined the two together.
Printed at 100% with no supports. Fit my Nerf Rival Perses.