I read a suggestion somewhere a while ago... Measure the diameter of your toilet bowl... then go to wal-mart (or somewhere equally cheap but less evil) and find a large bowl (roasting pan; tupperware; metal) that has a rim around the edge and has the same diameter as the toilet bowl. (And isn't too deep to fit in the toilet.) Maybe there are even oval-shaped bowls that would fit exactly, but if not, the rim on the round bowl should hold it in place.
Then, just lift up the toilet seat, stick in the bowl, close the toilet seat, and fill the bowl with litter. So the toilet seat sort of keeps the bowl in place. That way, you can also remove the bowl when you have to go. After the cat uses that, just keep reducing the litter in the bowl. Eventually, just put newspaper or something in the bottom. Then, I think the person said to put a little water in the bowl so the cat gets used to the sound of the water.... After that, take away the bowl. (And burn it

)
Ok. I just found a link to where I read about the above:
http://www.karawynn.net/mishacat/toilet.html
When I tried to get my cat to use the toilet, I put shelving paper over the toilet bowl (it sticks well) and covered that with plastic wrap, then put down the seat, and added litter. My cat wouldn't have any of it. And, I couldn't use the frigging toilet. So I gave up.
One last idea -- if you're not going to be using the toilet during the cat-training period, couldn't you drain the toilet, put a trash bag in it and over the outside, put down the seat, and fill the whole thing with litter? (And rake manually as needed...) Then remove the bag of litter after the cat has learned?
There's also this:
http://www.catseat.com/, which is really cool, but really expensive.
I've also thought about buying a cheap toilet seat, and sort of mounting it to a plastic container and using that as a litterbox for a while to get the cat used to standing on a toilet seat... Never tried it, though.
Good luck...
-e.