The class rules state that sitting out aids are of unrestricted design, except that they must be removable and can't extend more than 5' from the centreline. Designs differ a fair bit, but below are some pictures with dimensions posted by Peter M which provide a good basis to work from.
If you are adding a sliding seat to something other than a MiniSprint MkII (where its position is fixed) then Stuart Preston recommends 6-12" behind the daggerboard.
Image
