I did a lot of experimenting with floaters this past season. Look for a thread titled "Need help with colonies". Several helpfull tips in there.
The size and shape didn't seem to matter but floaters need to be covered.
Apples had the highest success rate over carrots, parsnips and potatoes.
Late season with a gland lure really helps but this is also the same time you will start to see scarred flesh do to the mating rituals.
Make sure your design prevents the floater from having any air between the bottom of the platform and the water. I had too much flotation in one of mine and had a live rat to deal with.
Like many of us have experienced, I am sure you will find that floaters are not as consistant as colony traps. They can really be hit or miss. It is however in my opinion another necessary tool to have in your tool belt.
Hope this helps