Graham,
At $40 you're being ripped off. I made some out of some old rubber strips glued together. I just cannot understand the price of these things.
You may find that yours don't even touch the frame. Mine don't, although I didn't find that out until later. I only changed one on one side for that reason.
As for doing it without taking the canvas off, I think it would be a colossal pain in the arse. I did mine with the roof frame off the car; it you wanted to change them with the roof on the car and the top on the frame I think you'd have to remove the chrome caps at a minimum (which isn't hard). You should be able to get an idea from looking at your roof now. There is a tiny screw or two on the underside that you would need to remove.
Here's a photo of the piece in place. Not a great shot, but the piece can be seen clearly.
James