Maxwedge. your idea will work just fine. I have used bonding adhesives many times on steel, aluminium, and fibreglass. You just have to pick the best, (most appropriate) kind of "body panel bonding adhesive". SEMs and 3M both make excellent products and I would get some guidance as to what exact product to use from a good reputable auto body products supplier. But you can look them up on their web sites and read there product descriptions and applications. i don't have the number of the SEMs product that I use quite regularly, but it is a brown, translucent colour and set up with in 15 mins to 1/2 hour. Some will set very fast almost like body filler, and some will take from 3 hours to 24 hours. The only caveat to using these bonding adhesives is that you should use the twin tube dispenser for the product. It is critical that the mix is right. The twin tube dispenser is expensive and also the product itself is expensive. About 35 to 45 dollars per product.
i would not trim the opening much larger, just make it kind of even and maybe a little hammer an dolly work to level it. Don't get near the trunk hinge. When you are hammer and dollying the metal, don't do it too much because the hammer work with the dolly directly underneath will stretch the metal then it will want to buckle and nothing will fit. On the piece you are going to implant, just make it about 1 to 1 1/4" larger than the opening you want to fill. That amount of overlapped flange will be sufficient for the bond to hold. Conventional wisdom says that an adhesive that takes a long time to cure is strongest. But I have found that the longer curing adhesives are troublesome. It seems I have waited a day or more for them to set hard and then still had to resort to using a heat lamp to get them hard. They are a pain. The medium ( 15 to 30 min) adhesives work best in my opinion. If you use the right adhesive and it sets fast and hard you probably will never need to weld it.