Do you still have the part that went in the top? What is it? You should be able to use that as a frame.
Cut out everything that would keep the Xenarc from occupying the space where you want it, as if the plastic was still installed in the dash. You should have to cut out a hole big enough to get your hand into, in the middle. Make a wooden frame approximately the same size as the Xenarc. Brace the wood frame well with wood dowels or small pieces of wood to the plastic frame using a hot glue gun or super glue.
Get some spray glue. Spray the wood frame, and the backside of the plastic piece. Get some grille cloth (color does not matter) stretch the cloth over the wood frame, wrapping it around the back of the plastic piece. Not too tightly! you do not want to warp the plastic out of shape.
Trim as much of the excess grille cloth as possible, ant test-fit the piece. Make sure that it is not warped out of shape, and that it will still fit. remove the piece from the car, and brush fiberglass resin onto the grill cloth, making sure that it is soaked through. Let it cure, and when it is hard, you should have a general idea of what it's going to look like.
Now, remove any bracing that you had to use to align the wood frame initially. Be careful. The resined grill cloth is not very hard, and can crack on you. From the hand-sized hole in the plastic piece in it's backside, you should be able to see the inside of the grille cloth. Build up the grille cloth with fiberglass mat and resin with two to three layers to build strength.
Test fit the piece. Get some epoxy glue and glue the faceplate of the Xenarc to the wood frame. Once that has cured, Get some autobody filler, Bondo, or a better brand from a auto-body supply store (easier to work and sand). Take the filler and fill in the sharp lines between the edge of the Xenarc and the wood/fiberglass. Try to smooth it out as much as possible.
Sand the entire thing with 220 grit sandpaper, then 600. Now, there are two ways to finish it, make it look like the black plastic below it, or like the vinyl around the gauges. For the plastic look, prime the piece well, using several coats and sanding with the 600 grit paper, then paint it using a matching satin black. OR, for the texture method, get some SEM texture spray #39853:
texture it, (practice first on another scrap piece, using different techniques to get the look you want) then after its dry, hit it with some 600 grit, or steel wool to "flatten" the top of the texture. Color it using a matching SEM color coat:
The texture should look like this when done: