2x4 frame (file 8a) as well as single cell option for the 1.5in / 40mm rock wool starter cubes.  Allows your cubes to be in contact with heated seedling starter mat until roots protrude, and then second frame lifts 20mm to allow safe root growth until you transfer to soil / hydro as you prefer.  As you can see, works great - have used it for many different kinds of seeds with great success.  No supports needed, but you may find a brim helpful for the frame as it has such a small footprint on the build plate.