if you want to buy the jig, I like to use my CRB7 router jig. It will flush trim, has an offset knob handle, and it can do many other things as well, if you buy it with the options it will do more than the 7 functions of it's original design. Like centered Mortising, laminate trimming, small circle cutting, very large circle cutting, etc.
For making just a router base with an elongated shape and a handle off to the side, I made my own by tracing the original router base outlines onto a piece of 3/8" clear Lexan (Lexan won't shatter or crack) and then took a small can and drew around it to make the curve on the elongated area. Then I just drew in the straight lines between the circles, cut it out on my band saw, and smoothed the edges on my belt sander. I then drilled holes to match the original router base and one hole in the small rounded area to attach a handle. You can use any kind of handle that you like. I chose a large cylindrical handle from the gray drawers at Lowes for my offset handle.
Central North Carolina