Thwart Adapter
All you need are two pieces of 1 inch thick plank which are tapered to fit into the aluminum tube (2" diameter, 0.125" wall), and some bolts. Any hardwood will do. Clamp the assembly to your canoe first and test for balance before committing to final positioning.

Adapter Sketch

A Tump Attachment Scheme
(Could attach to thwart-gunwhale screw in foreground)

Tump,thwart, and arm rest rope

Tump Material
The tump can be anything that does not absorb water or stretch. Leather or nylon strapping work well and a good width is 1.5" to 2". Dollar store nylon strap or an old seat belt will do. Aluminum tubing can be found in Canada and the USA at Metal Supermarkets. I choose them for convenience, not price.
Thanks to Sarah J. for pointing out that some people prefer to attach the tump to the gunwhale an inch or two forward of the thwart. I gave this a try and thought it was more comfortable but also found the canoe's balance altered on inclines. The moral is to adjust to suit yourself.