Mine is an afc in there instead but the same idea,also not the quickest way either
I cant remember exactly but it was something like this...
Remove the vents and any plastic (if there even is any?) around the entire opening so what ever you put in there sits flush against it.
I then used an old glovebox front for the slight contour I think, then heat gunned it to shape a little bit then cut to fit afc and dash.
Then brought a thin flexible sheet of carbon and cut to shape again and epoxy resined it to a well scuffed glovebox part above.
Then siliconed the afc into the tight fitting hole, and siliconed into the dash.
I also glued apart over the old heater vent hole aswell as it can get hot behind there.
Things I would of done different is proberly used a better quality bit of carbon with maybe a 2x2 weave instead.
If I remember right I think the avcr fits better in the 3 gauge hole,i think the lip around the empty gauge hole almost clips into the join/seam around the middle of the avcr holding in place,or I think it did on mine.
I think I may have still siliconed it in but then you only have to make 2 small pieces to fill the rest of the hole in.
Then sod of the stereo and fit the 3 gauges in there,which once again I think almost just pushes/wedges
in there quite nice on its own with out any brackets.
Heres some pics..
The join where the 2 parts are epoxyed together,took a while for the glue to set and need pegging together with some strong clips but seems to be holding fine.