I personally think that seedbags are too small, especially if you have full nature.
So what I thought of was a seed sack, or something. Basically it would be a 1x2 object made out of 8 linen cloth, and 4 strings. Inside would be a 4x4 space (like an inventory without any backpacks, merch robes, etc) for seeds. It would act exactly the same as seedbags, just with more space.
The reason I thought of this is because most of us have large fields, and even an inventory full of seedbags may not be able to tackle the whole thing in one harvest.
Any better suggestions are welcome.