New engrams for these variants are available at the same level as their large counterparts.
Small platform: Has a width of 2 walls and consumes 1 building kit. Can be placed on *some* redwood trees that would be too small for other sizes.
Adobe sloped cellar door
Medium platform: Has a width of 3 walls and consumes 2 building kits
Can be placed same way as regular cellar doors, uses roof item instead of ceiling.