AlterEgo with the voice Daisy installed is a free voice synth that can act as a placeholder (it needs a DAW to run though).
The closest thing possible - as far as not using pre-made midi or making a UST from scratch within UTAU (GG, NO!) would be to either use Vocaloid, create a VSQx, and convert it to VSQ to use in UTAU (but this is out of most young users' price range if you go through legal methods and some elements in the vocalsynth fandom are very vehement about piracy of the software, even for private use that's never published), or use a DAW that's capable of midi creation and using AlterEgo as a VST.
