trackball mounting plate and plexi

[severdhed]

i need a little help with this. I am getting ready to finish the panel on my showcase cabinet.  right now it is just a simple 4 player panel, just bare MDF currently.  i plan on picking up plexiglass sometime this week to make this look less crappy.  anyway, i left enough space in the middle of the panel to add a trackball.  i have a Utrak in my vertical cabinet, it was very easy to mount and to get the plexi cut to fit.  (i just used the 3 1/4 hole in the MDF as a guide for my flush trim router bit to trim the plexi, it fits perfectly).  however i dont really want to buy another Utrak since i already have a 3" wico trackball collecting dust in my basement.  i do have a mounting plate for the trackball.  i plan on using a router to recess the plate so it sits flush with the top of the MDF, and then covering it with plexiglass.  i'm not sure though how to make the trackball hole in the plexi.  the mounting plate is pretty thin...will it be thick enough to use as a guide for my flush trim bit?...if not how else do i do this?  the only other way i could think is to use a 3" hole saw in the MDF, use that hole to guide the router, then cut away the rest of the hole and recess the mounting plate.  i'm just worried that i wouldn't get it lined up perfectly. 

maybe there is another way that i am missing....any suggestions?