what brand is your router?
What router do you have that weighs in at 18 lbs?
My old Porter Cable 5182, about 1980's vintage ... is a monster at 18 lbs also. Is that the one you have?
I would use nothing less than 3/8" aluminum for a insert plate.
Mine is only used as a hand held because the large diameter motor won't fit most lifts. The new Jessem MastRLift 2 have cam inserts for different size motors that may allow for a larger motor than a 7518, but I haven't tried it so far.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 07-25-2015 at 12:11 PM.