If you're handy with some hand tools you could modify an inexpensive shallow aluminum Weaver
pica tinny rail for a shotgun barrel mount. I cut one at an angle to match the length of the slide base ahead of the trigger guard then filed a contour to match the surface and curve of the trigger guard front. Using a permanent 3M VHB bonding tape I attached it to the 469. The thickness of the tape changed the curve of the rail to match the curve of the slide base. Be very careful in lining up the rail with the slide base so the curves match. Once it's pressed in place it's, as the spec on the tape says, permanent. The only way to get it off is to separate the adhesives and lots of hard work with some acetone to help! Use soft jaws to prevent marring the edges of the pica tinny rail while you're filing the contours. It took me about an hour with constant trial fits to complete the shaping. Once installed it looks and works like it was made for it. Any bright spots were touched up with a black magic marker.
There you have it! A custom pica tinny adapter with no drilling and tapping!