As you said, you can buy a good machine but it will be expensive.
Here is an idea that many will find hard to believe, but it works and costs next to nothing!
How well it works depends on how exactly you implement it.
This solution requires a bit of space and a solid foundation - it will not work for apartment dweller.
Imagine 2 basins filled with water to the depth of say, 1.5-2 feet. The kayaker sits on a seat (any seat) between the basins and paddles just like a regular kayak. The difference is that in a kayak you plant the paddle in water and move the boat. Here you are stationary and the paddle moves through the water.
The key here is the adjustment of the resistance level.
You can use very narrow blade (greenland paddle should work) but then the water in the basins need to be fairly deep for proper resistance. On the other hand you can have shallow water and have a wider blade.
You cannot use regular euro blade - the resistance it creates will be too much, unless you want strength training.
Other details to work out is the position of your body and a way to keep you firmly in place (so you don't slide when applying the pressure)
Larger basins are better than smaller, the water will move violently so you will need some distance from the water level to the top of the basin.
Again, all these are details to be worked out.
I used similar setup many years ago but I used only one basin and single-sided canoe paddle.
Sorry to hear about your cuff. How long ago did you tear it and what kind of surgery did you have?

: Has anyone ever cobbled together an off-season paddling machine? The store
: boughts are thousands of $$$$. I'd be very interested, seeing as how I had
: major rotator cuff surgery to my shoulder a few weeks ago. Doc says no
: kayak till October! This is my third repair, so I think I'll do everything
: he says this time. That's why I decided to build a boat if I can't paddle
: one. My plans from Orca should be here any day. I'm pretty excited about
: it.