New and in stock at Get Reload are these Element Ergo-Style Pistol Grips for WA and WA-clone Gas Blowback M4. The bottom can be opened as inside there's a hollow space for you to use as storage such as spare batteries for your optics or comms. Material is nylon-fibre, available in black and dark earth, and cost US$20.99.

Element Ergo Style GBBR M4 Pistol Grip - Black

Element Ergo Style GBBR M4 Pistol Grip - Dark Earth