Bootstrap Farmer Drop Seeder https://www.youtube.com/watch?v=MTVvjtp3V8E&t=5s
Incredible Farm – Customised Seeding System https://www.youtube.com/watch?v=ZzWQbwlgUmA
This simple-to-make tool should speed up seeding cell flats quite a lot. https://www.youtube.com/watch?v=nvCdRpIj6xE
IMP Seeder 72-Drop https://www.youtube.com/watch?v=Ub2kB4UYH-w