Madbull have now have the PRI licensed Gen III Round 9 Inch Rail made especially for any M4 Carbine CQB length hard guard. It is a comfortable, tube shaped free float rail completely made out of Carbon Fibre making it fragile but lightweight!
They come in 3 different colour camouflage. Black, Olive Drab and in TAN so it really depends on the environment you more often skirmish in. It comes with 4 additional metal rails which you could apply to the 12, 3, 6 and 9 o'clock of the hand guard so you can include any accessory you'd most likely add to the gun.
You want customization? This will give you customization!