Damage Industries just released their forward grips that can be mounted on rails. You get to choose between the Knob and Grenade styles... "Constructed of 6061 Aluminum with Mil-Spec hardcoat anodize. Non-protruding clamp held with stainless steel combination head (phillips / slotted) fastener. Reduced weight - cored out internal storage area (accessible by removing grip). Made in USA."

Damage Industries have this Enhanced Combat System Furniture Kit comprised of the ECS stock, rail covers, and grips. The stock remains you of the CTR trock but this one's more curvey like the contours of streamlined human body. For a kit, it will cost you US$136.60 and available in 6 colours. Most probably that this will fit mainly airsoft gas blowback M4s as it's designed for the real steel ones.