TDI rail mounted on the plastic handguards.  For mounting a light it works well enough.

Quoted: HEY I really like that. how did ya do that? did you just drill holes and mount?

I did that on another AR.  For the above M4 and another one I have I did the following:

- mark the ribs up about 1" from the edge of the handguard.
- using a dremel with sanding drum and sand down the ribs between the edge rib and the mark
- remove inner heat shield
- Mark and drill holes
- Install the rail
- reinstall the heat shield.

Doing it this way takes a bit longer, but the results look much better IMHO.

ETA - with the TDI rail and a M4 (Cav Arms type) handguard the rail is parallel to the bore and you can position the light all the way foward, or all the way to the rear (allows for thumb activation with a magazine well grip).