for fluffy grass seed a drop seeder most likely will work best, the small hard seed a regular or small seed attachment is most likely best, or a small broadcast spreader,
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
for the DIY builder most likely the biggest difficulty is the metering system.
I came across a few simple units that others have made or are selling, and will post below for some possible idea,
this is a garden planter that uses dimples on a shaft as the meter system and a brush,
Four-row Pinpoint Seeder - YouTube
this one uses a cupped disk, much like a elevator cup, would work for fluffy seeds and large seeds,
Seed Drill - YouTube
I thought this was creative, plastic bottles for seed reservoirs and a wood frame
http://www.mytractorforum.com/showthread.php?t=69850&page=2
http://www.mytractorforum.com/attachment.php?attachmentid=56820&d=1229135274
crustbuster uses on this system a slot
http://www.crustbuster.com/images/no-till-drills-and-planters/Wobble-Slot-Single-Seed-Placement.jpg
I have thought the paddy seeders were ingenious
http://www.ksnm.co.in/upload/blog/51815direct-paddy-seeder-1010813.jpg
a very simple concept and could be adapted to a furrow type with a funnel type catch under it and a drop tube,
the old wheel barrow seeder,
Digitization Projects Philologic Results
pictures
http://lincoln.lib.niu.edu/fimage/lincolnimages/ardrey025a.jpg
how the seeder worked was a thin wood plate in the bottom of the box that shifted right and left, moving the seed over the hole to drop, the arms one can see, hit the spokes that shift the plate,
another design used a small disk that was bent right and left, and it had a arm that would shift the seed plate with wobble disk, (much like this wavy coulter,
http://www.gwtillage.com.au/images_product/coulters_wavy.jpg) my guess is there were different disks one put on to adjust the seeding rate,