The fondant sits in an apidea super on top of the stack.
To stop it dripping you can wrap it in a plastic bag and scrape it with a sharp knife a couple of times at the bottom. It just sits on the top bars.
I have also tried it sitting on a piece of plastic queen excluder cut to size and that works as well.
You could use an empty apidea instead of a super and just tape it in place but the super is a more secure arrangement.

There are loads of design faults with Kielers.
I don't own any myself but have worked with them quite a bit as some members of our queen rearing group use them.
The guy who had the most - 15 - has started to change over to apideas.
They need far more bees than apideas
There is no inner cover. You have to make one or half the bees escape when you are inserting a ripe queen cell.
The poly is much lower quality - determined bees can chew out even with an excluder fitted over the front.
The sliding floor underneath often leaves a bee sized escape route which is not good when transporting them in a vehicle.
The mini frames are just top bars with no sides so they often get stuck to the sides and if you are not careful can get pulled apart on removal.

There was a mini nuc comparison thread on the old bbka site.

http://www.bbka.org.uk/members/forum.php?t=4699