Just a suggestion that made removing the seeds fairly easy for me. I took a flat surface with a lip on it on 2 corners at least. (I used a long standing mirror). Since I didn't care about maintaining the consistancy of the buds, as my goal was to just get the seeds and pretty much toss the shake as it contained too many non mature white seeds to make smoking it any fun. (maybe an edible would have worked)
Anyhow all I did was I raised 1 side of the mirror up, so it sat on about a 5 degree angle towards me, maybe 10 degrees at the most. After that I was able to just roll the buds in my fingers breaking them up and the seeds would drop and roll to the bottom of the mirror, where the plant material would just stay directly below my fingers. This worked quite well for quickly removing the mature seeds and once I had a pile of ground up biomass on the mirror I would just stir it around with my hand/fingers and the mature seeds would roll out of it, the immature ones would stay with the shake.
Worked pretty well for me and I used dried buds, very dry made it easy to crumble them into tiny bits.
I would say this method probably collected a good 70% of the mature seeds and only about 30% of them I had to pull from the biomass.