Doors and Windows - Attaching Wide Extension Jams

I've been installing a few Andersen Casement Windows every year in my 1970 three level home. The upper levels are 2 x 4" construction, but the lower level is 2 x 6" construction. The extension jams are too wide to nail through the width as I did with the ones for the upper level. I can't toe nail or pocket hole due to the frame members. For some of the rooms I have made my own extension jams out of cherry and walnut so I don't want to mark the interior surface of the jam either. Any ideas out there?

Drill 1/4" holes in the edge of the window frame 3/4 - 1" deep. Drill the same in the jambs so that they line up. Apply a few drops of glue in the holes & insert dowels to connect. Before applying glue check the fit.