Update: Day 19: Separatist Shuttle

Day 19 is more likely the Separatist shuttle, also known as a Nemoidian shuttle, or Sheathipede-class transport shuttle.

This really makes more sense, as the shape of yesterday's build follows more closely the lines of the Separatist shuttle, not the AAT.

Lego Separatist shuttle:

Episode I Separatist shuttle:

Thank you to Word Geek for pointing this out, and for fruitlesspursuits.com for their post as well.

Wookieepedia entry for Sheathipede-class transport shuttle.