I bought a product called CitiKitty. It's a tray with ridged rings that fits nicely down in the toilet under the seat. It was about $30 I think and then we had to buy the flushable littler at $15 for 10 lb. Figured that wasn't too bad of an investment if I can get rid of the litter box. You start out with a decent amount of litter in the tray for at least a week until they get used to it. It's a bit of a pain to take in and out if you have to share the toilet with the kitties but we only have 1 bathroom downstairs and that's where I had initially put their littler box anyway, which had to be removed.
Anyway, after they are used to jumping up on the toilet to go, which the kittens took to right away, you wait a week and then cut the inner ring out and start using a little less litter. Trust me, you'll be using less anyway because they scratch it all out into the toilet, over the seat, etc. It's a bit messy but, again, I can GET RID of the litter box hopefully so it's bearable for a few weeks.
Each week you cut a ring out, there are 5 of them, and cut down on the litter. We're on ring 2. They are pretty much doing #1 into the toilet now since they always gravitated toward the center anyway, and we get a few #2's in there as well. We've had 3-4 accidents, once cause I hadn't gotten the wet litter cleaned out before the next time somebody wanted to use it. Once for sure since I witnessed it was because they both wanted to use the potty at the same time and Mork basically used his size to knock Mindy off and out of his way. Once because a human was occupying it and Mork needed to go. I think there was 1 other time but it was in the middle of the night so... At least the accidents were in the bathroom next to the toilet on tile flooring so they were easy to clean. I think they are doing pretty good, at least as good as a human toddler being potty trained.
Oh, if you do get the CitiKitty product, it also includes a bag of catnip that you are supposed to use to entice them up to the toilet. DO NOT use this unless you want a HUGE mess to clean up. Mork went bonkers and there was litter EVERYWHERE! At least it hadn't been used yet and was clean.