Find a Parker standard trigger guard and modify it.
What mc says. But original straight grip Parker trigger guards had a much longer tang than the much more common pistol grip trigger guards.
Several years ago, I contacted the source KDGJ recommends for a few safety parts for a 12 ga. 1 1/2 frame GH Parker, and couldn't believe his crazy asking price. A short time later, I found a whole pile of Parker internal action parts that included everything I needed and much more on E Bay, for a small fraction of his price. I follow and buy enough double gun parts to know when prices are fair and when they are wildly inflated.
Another good option is Gun Parts Corp. in West Hurley N.Y., but you need to keep checking back if they are out of stock. They are always getting new stock in. No matter what you do, it is going to be a lot tougher to find one with DH engraving, so you may need to factor that into your costs..