After playing a couple of games with my M4 AEG I found that the plastic handguards were just not sturdy enough for me and looked a bit too stock.
I then sat down and took my M4 apart and studied the Delta ring and gas tube connection, after a while of measuring and searching Google for reference images I came up with this 10" RAS Keymod Rail.
It attaches right onto the threads on the upper receiver and just screws tight, and it can be held in place with the gas tube. Alternatively you can use a rail riser or optic as I have done in the pictures to keep slight pressure on the rail as not to unscrew (it already is fairly tight).
The rail is actually free floating, but I designed a mounted suppressor that screws OVER the outer barrel extension to give extra strength to the rail and keep the inner barrel secure.
The suppressor has standard 14mm CCW threads.
For adding rail segments I suggest my keymod rail segments design.
Make sure you print it in a sturdy filament like PLA or ABS, others might not hold up to the weight of the rifle and might sag/bend.
Also make sure your cooling and retraction settings are properly dialed in.
Mounting the rail
Disassemble your current rail/handguard so that only your receiver and outer barrel are left.
Screw the rail onto the threads of the receiver whilst slightly unscrewing in between to loosen the threads of the rail a little.
Once in place, mount the gas tube or optic to keep the rail in place.
Lastly mount the suppressor onto the outer barrel and into the rail by just simply screwing it on.
I have just also uploaded a 7" and a 12" version alongside of the original 10", for those that have a CQB length airsoft replica and for those that have a rifle length barrel.
I have on special request of Phreqq made a 4" version for those that have no need for their barrel to compensate anything. ;-)
I also made some changes to the threading section on mention of simonvez who told me that some barrel types don't fit great in my original design, thus I changed the barrel opening to 24mm on his recommendation.