'Duck, towing gliders often needs a modicum of speed. Not a lot but more than a pure slow flyer would provide. Don't spare the power. But add plain flaps for drag and a bit of extra lift for slow flying.

For a true slow flyer build light, use an airfoil with at least 5 to 6% camber and at this size stick with something around 10 to 14% thick. The 6409 is nice but the 6412 at this sort of chord would likely work a touch better. And then there's my own favourite the Goettingen 501 as used on my 84 inch Flamingo old timer. I've often been amazed at how slow it can fly yet how well it can penetrate for the sort of polywog shape it has.

But if you're going to tow gliders, especially the faster flying jumbo scale types, then stick with the ClarkY and add the flaps for quick descents.