Ok, lets see. As item type we need a seed. Using plant would get us a plant made out of seed material, which would be weird.
Looking in the ever so useful string dump I found item type SEEDS. Item subtype shouldnt be needed.
As material we need to designate a plant type from the raws, so that would make it PLANT_MAT:MUSHROOM_HELMET_PLUMP
Then as material subtype we need the seed material defined in the plants raws. Found the following:
[SEED:plump helmet spawn:plump helmet spawn:4:0:1:LOCAL_PLANT_MAT:SEED]
Clear enough, lets use SEED as material subtype.
So, the result: [PRODUCT:100:3:SEEDS:NO_SUBTYPE:PLANT_MAT:MUSHROOM_HELMET_PLUMP:SEED]
Well seems you had that one except for the item type being SEED instead of SEEDS.
Skulls seem like a tricker one, I made some attempt at it when I tried to make golden skulls, but failed.
I cant find it as an item type or a material type in material templates. I did however find this here:
[BODY:SKULL]
[BP:SKULL:skull:STP]
[CONTYPE:HEAD]
[INTERNAL][SMALL]
[CATEGORY:SKULL]
[DEFAULT_RELSIZE:200]
This lets you make a totem out of the part. The relationship between the skull and the brain is established within body detail plans or the creature itself.
[TOTEMABLE]
From what I can tell from the body part raws, the skull is much like all the prepared organs you get when butchering a creature. Those are meat with a specific name, skulls are bone with a specific name.
What confuses me even more is that there is no item type for bones either, just one for totems.
I have found ANY_BONE_MATERIAL. This could at least let you select any item made of bone, but how you could keep it from using +turtle bone bolts+ or regular bone instead of skulls, I wouldnt know.
At least not without reworking some basic raws, for example by adding a special REACTION_CLASS tag to the material that the skull uses. But im no expert on that; I dont know how to do that without having to change every single creature in the raws.
For now it might be easier to just use totems, [REAGENT:A:1:TOTEM:NONE:NONE:NONE] should do the trick.