I am thinking take the evolutionary path of a slime mold to it's logical conclusion. I am thinking he may be silica based as well. Something Bashir said in the first episode that had Dr. Mora in it.
I could see a changeling evolving a mimic ability to look like solid creatures to fool the other liquid slime mold type creatures that were predators.
Maybe they pick their human form after the Progenitor Race because they decoded the DNA in the Gamma Quadrant years ago and since they were the "first intelligent species" in the galaxy, the changelings picked that shape as a "tribute" to the first life that seeded and "controlled" the destiny of the other species, something that fits the changeling manifesto, the "controlling of other race's destines" part specifically.
Either that or the Progenitor race came across the slime molds in their pr5imordial state and added some DNA like stuff to them, an unfolding program, and it made their "natural humanoid form" look like a progenitor. Since they may not evolve as quickly since they simply change shape to evolve the "Progenitor DNA" might be more expressed than it was in a Klingon or Romulan etc....