I was wondering if anyone out there has done this. I'm wondering if there would be a problem down the road because of the different material and the expansion and contraction rate. Plan to glue it in with panel adhesive. I've used it on other projects with great success but that was always metal to metal.
Cut the metal flange so it fits flush with the glass surface leaving a small gap around it before you set the flange in make bonding stripes and glue them to the bottom side of the cowl before ya install the flange. That gives the strength ya need After the strips cure, bond your flange in and finish off. use a good adhesive made for bonding glass, Your local paint supply will have it. If done correctly it will last the lifetime of the car.
Methacrylate adhesive for metals/fiberglass bonding. http://www.materialstoday.com/compo...-use-of-structural-methacrylate-adhesives-in/
Sounds like you got it sorted. I'd keep the opening as precise as possible. The only issue might be a moderate amount of shrinkage on the finish surface. The closer it fits the less any of that will matter since it will be small, or even possibly in the trough area. Panel bond is a decent choice, the suggestion of glass setting urethane has merit too. Bad ass stuff that holds forever but shrinking of final body work is still a consideration.
I've yet to follow through yet but my plan is to glass in this aluminum trunk riser in my Wescott roadster. My intention is to completely finish off the underside and not have it start cracking from not using the right process. When I called Carl Wescott for suggestions on how to proceed, the first word out of his mouth was methacrylate adhesive. He offered to help with it if I brought the body to their shop because they use a lot of it but I no longer have a trailer. My plan is to purchase the two part adhesive but it requires a special dual tube gun to apply it and I don't expect to do any more of this type of work so hopefully someone near me might have one when I finally get around to it.