A plyometric drill I've used to improve seoi.
Start in a normal stance, leap forward about 2 feet, give or take. As you leap, turn 180 degrees and drop into a full squat (butt on heels). Straighten your legs, without moving your shoulders - that is, your shoulders do not rise as your legs straighten, you bend at the waist.
Stand and repeat.
Sometimes this is done leaping over a partner; sometimes it's trained in steps (leap over with just the turn, no squat, practice squat with no leaping).
Sometimes I have students tuck and roll forward as they straighten their legs.
It doesn't replace uchikomi and nagekomi with a partner, but does help you become more confident and explosive in your movements. I had one student struggling with ippon seoi, did this for a month, pulled off a nice explosive seoi in randori - I wanted to get him to a tournament, just to hear the crowd if he could pull it off. Unfortunately, he started competing MMA shortly after and got away from seoi.