This conversion is "convertible": it allows you to switch between single-drive and double-drive quickly, without having to dismount the mother-of-all. It does require a screw-driver to loosen (and retighten) 2 screws. So that's a big advantage over the method I first tried. The other big advantage is that you retain use of the tension knob in this method.
In order to do the convertible conversion, you need to replace the flat piece of wood that your Traddy's MOA rides on, with a new piece that you have to make yourself. I am not going to tell you how to make it; if you don't have woodworking skills (sawing, drilling) and/or tools, you'll have to enlist the help of a friendly carpenter. In my case, I called on my father!
[replace existing MOA slider - bottom - with new MOA slider - top]
The new slider piece should be the same thickness as the old one - about 5/8". It's OK if it's a little thicker, but don't make it thinner (or you'll find you won't be able to tighten it down properly). It's best made out of plywood, because it takes quite some stress and the screwholes are quite near the edge. Plywood splits less readily than solid wood.
The dimensions should be: 11" long, 2" wide, with a 1/4" slot. The slot stops 1/2" away from the short edges and is 1/2" away from the long edge.
Mount your MOA to the new slider using the old hinges. I've put them at the end, where they used to be, but this doesn't matter: you can drill new holes and put them in the middle of the MOA. Whack a new thumbtack in for the tension knob to rest on.
That's it! Now you can slide the MOA back and forth into SD or DD position by loosening the two screws that hold the slider plate into position.
[MOA in the "single drive" position]
[MOA in "double drive" position]
This works really well. Actually, I don't understand why Ashford doesn't provide such a part. It's dirt-cheap and means you don't have to buy a new MOA.
Now, I have been playing with this a bit, and I am getting better at using the DD setup. Because it is much faster than my old SD 10:1 ratio, it takes a bit of getting used to...but now I am back to treadling at 1.5 strokes/second (about 90-100 turns of the drive wheel per minute, which is "comfortable-fast"). The Traddy really starts to vibrate at these speeds, so I suspect that a lot of my energy is getting lost. So here's what I've done to dampen the vibration:
1. clamp the MOA and the slider bar together, near the frame of the spinning wheel, thusly:
[clamp the MOA and the slider bar]
2. instead of using the tension knob, use a folded-in-half old mouse pad to regulate tension. I think using a wide wooden wedge or doorstop would be even better, but I don't have one.
[instead of using the tension knob, use an old doubled mouse pad]
These two things have really lessened the vibration and allow me to spin at maximum speed.
If I want more speed, I will have to start making other modifications using custom parts.