use pantyhose and hot glue. stretch the material over the surface to the outer edges of the vent. (i assume you've already secured the PVC). paint it with resin and let it cure. Don't put resin where you plan for the gauge to be or you'll make more work for yourself. Just enough to make sure it sticks to the lip of the PVC as well as the edges of the vent. When the resin cures, start applying your chop mat, dynaglass, dynatron, duramix whatever to the INSIDE of that delicate pantyhose shell you have. When your reinforcements are dry, you won't have NEARLY as much work to do on the outside, plus your shape stays pristine for the most part, and after trimming off the glue, you'll only have minor feathering to do with your favorite filler.