I couldn't find a model of a metroid, and I've been trying to learn how to sculpt, so I made one. The design is based mainly on the version from Metroid Prime, since that was the first Metroid game I played and I like how they look in that iteration.
I sculpted it so that the membrane is a separate hollow piece, so that you can print the membrane in clear resin and (hopefully) see the nuclei through it. I've also included a solid version for if it won't be printed transparent. There is a mouth sculpted onto the underside, which I added so that there would be a convenient point to add a flight stand.
I've included the supported files, exported as STLs. They're scaled to be 32mm, if on the larger side (but their size in the games isn't exactly consistent, so I think they're fine). Plus, any smaller and the excessive detail on the nuclei wouldn't be visible. Honestly I was surprised it was visible on the test print.