I have used the supplied fiberglass panel in my 320 (over 600 hours) without distortion.  After the forward side (non-gel-coated) was sanded and evened out,  one or two bid (I can't remember which) was added to stiffen it and to make drilled holes finish cleaner.  The blank panel was then located and fiberglass "tabs" were built on the fuselage with the panel clamped into place and release taped.  The panel was glassed in after all holes were done and paint was applied. 

 

Using the instrument template available from Aircraft Spruce and after fine sanding of the panel face, the layout was carefully done in pencil by first drawing a grid.  I did use step drills (unibits) for up to 1 inch holes. 2 1/4" and 3 1/8" holes were done on a drill press with wood backing and good quality hole saw bits.Each succeeding hole done with greater trepidation for fear of shattering the whole mess.  Square holes were cut with a dremel tool and filed for final fit. The radio stack trays were mounted to aluminum straps that were tapped to accept flat head machine screws used to hold up the trays.  This setup was "released taped" and flox fit into the radio stack area.  Four holes (two on each side) were drilled thru the sides of the radio stack glass and straps.  The stack straps were then taken out and tapped to accept machine screws that would hold the aft part of the stack.  The forward part of the stack is held by aluminum strap "legs" bolted thru the nose gear tunnel.

 

The panel was painted with a medium dark truck paint chosen after it was determined that light colors provided too much canopy reflection at night and black made it difficult to locate conventional steam gauges in the daytime.

 

I had to use the glass panel to fit all my stuff into, onto, under, over and behind it.  There are 25 major holes (over 2.25") and probably 30 minor holes (under 1"), not counting the many "screw" holes.

 

Have fun,