You can just turn the output down from your recorder, so that the meters on the recorder match those on magic lantern, I have done this and had good results.
Just be careful to have it turned right down before you plug it into the camera so not to overload the input.
You can make a pad cable, I used 1.8k Ohm, and 100 Ohm resistors, but anything similar will probably work fine. You're just trying to get the signal down a bit.
Put the resistors at the camera end of the cable, to keep the maximum signal level in your cable, this will reduce the effects of any induced noise.
It's quite tricky to get the 4 resistors into a tiny little 3.5mm stereo plug shell, but it can be done. Try to get 1/8Watt resistors or smaller if possible.
One advantage of the PAD cable is that you can split the output from the recorder top monitor that with your headphones, but remember if you change the output volume to adjust the headphones, this will also change the recording level in the camera.
I like to carefully set the output of the recorder (and the analogue gain on the camera) so that the level meters on the camera show red as the same time the recorder peaks out, this way you don't need to be able to see the levels on the recorder while filming.
Maybe I'll make a video tutorial about this one day.