Onderzeeboot
Ecstatic Orb
Nitpick: it’s “an artifact or creature”, see Makeshift Munitions.
I like this in theory, but I think removing three counters is quite a big ask. Comparing it to Experiment One, I wonder if the cost couldn’t be reduced to removing two counters (or maybe one counter and mana). I mean, removing three counters is losing an investment of three sacrificed permanents and mana. You would almost never pay that to make something indestructible while shrinking it considerably as well. I think this critter needs a little more love to feel rewarding!