I've never worked on a Mirage, but before making my own knifes I started by doing what you describe to about 35-50 Benchmades.
I agree that having screws from each side going into a metal spacer is the best approach, but if you want to go from one side to the other into wood, Micarta, or G-10, here's a tip: buy some thick-walled metal tubing about 3/16 diameter with an inner diameter smaller than your tap drill size. Then, drive this tube, like a pin, into a snug hole in the scale material (a little epozy won't hurt, though the tight fit should do) BEFORE shaping the scale. Then drill and tap the tubing. This gives you a secure metal lining for all of your screw holes so they won't strip; much better than tapping non-metal scale material.
For a nice fit, expect to profile the scales while they are on the knife, and remove a tad of the liners in the process. Shaping them "by eye" while they are not attached to the liners will never get as nice a fit.
Materials I had a lot of success with were aluminum, titanium, micarta, and Dymondwood, though the last one can chip or crack if the fitting process is rough. This is an easy and fun way to make your knife unique and learn a lot, too. Try to contour your scales instead of making them just flat like most production scales, add bolsters, or change the clip placement to suit your needs. The Mirage seems like an excellent choice for such treatment - if you have an idea, try it!
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-Corduroy
(Why else would a bear want a pocket?)