This is the story of how I made a Mycetic Spore. I know there are about a million Mycetic Spore articles out there, but I thought I'd contribute mine because I'm really happy with how it turned out, and it was dirt cheap and dead simple to make.
The materials you will need to make this are
1) One 6"ish styrofoam egg from your local craft store.
2) One block of Sculpey or Sculpey equivalent.
3) Some plasticard
4) One of those little Squamous Bits from Battle for MaCragge or the Genestealer kit (not absolutely necessary, but I think it is a nice touch.)
5) Mat Gel Base (apparently this is made by Vallejo, it's a gel substance meant to be mixed with paint to give texture or with collagey stuff)
After the Gel Base is on and dry, let's make the tentacles. To make the tentacles, you'll need to roll out tentacle shapes from Sculpey. My 'pile-o-tentacles' used exactly one block of Sculpey. Make a ring from Sculpey large enough to set the egg on and attach the tentacles to said ring, then flop the tentacles all over eachother in a big old tentacley mess. Squish the egg into the center of the pile of tentacles so that after the Sculpey gets baked the egg will fit nicely. Remove the egg and bake the tentacles as per the directions on the Sculpey.
Once the Sculpey has been baked, you can glue the Squamous Bit to the egg and the egg to the tentacles. Then cut out a base from plasticard, glue the tentacles to the plasticard, and you're done!
I painted the egg like I would the shell on my bugs, and the tentacles like flesh. The end result is about the size of a drop pod, large enough to hide a Carnifex behind, and looks pretty good (if I do say so myself). Mine of course looks something like an eldritch coconut, but that's just the way things go.
If anybody asks you to justify this design, I like to tell them that it's an egg full of biomass and instructions for assembling a unit, it births out gaunts or zoanthropes or whatever through it's little Squamous Bit.