Sadly my camera is being annoying right now (it's so old!!...) and i'm gonna try to fix it and send u guys pics ASAP, but for now a description:
Basically there were three relative positions i wanted to test, bottom of copter, 2/3 or so of the way up the copter, and maybe 3/5 of the way up the copter.
At each of these locations, i make a pencil mark, and i create two grooves equally space on opposite sides of the pencil mark (how far apart are ur spars?). Small grooves guys, my rotors happen to be 1/16 by 3/64 in dimensions at the spars. Make sure the grooves aren't too big. Might want to start small and sand to perfection.
Now, place your rotor into the grooves. Should fit fairly snugly. Take a small piece of balsa, and just cover up the grooves and surrounding area. Take some string, and create a long loop continuously around the motor stick. Tie it tightly and you done. If all is well, removal should be fairly easy and there shouldn't be any problems during flight.
Let me know how this works out if you try this. Pics will be up shortly.
Edit: For more security, tie a knot over the balsa "holder" piece in between the spars, and either side outside the spars. Also, if the fit of rotors is not completely snug in the grooves, place a balsa scraps or clay (though clay is denser, much more so).
You're only cutting a tiny, tiny groove. Plus you will be adding a piece of balsa on top, and the weight of the string. There may be alternative materials though
I would guess that sewing thread would work best. The way I would do it, is to trim with the string, glue down the best position, and then do a few more trim flights to double check to make sure it flies just as well without the string. lllazar's idea does seem to be clever.