With compact spare tires from the FWD Sienna almost impossible to find, and tending to cost upwards $150 plus shipping if you manage to come across one - I investigated the using the 16"x4" spare tire from the Chrysler Town & Country / Dodge Caravan minivans, which you can find in abundance at junk yards or on ebay for about $25 plus shipping.
The bolt pattern is correct, (5x114.3) the center hub hole is about a 1/2" larger (1/4" all around) so it fits over the Sienna hub but is not hub-centric. It also fits perfectly in the left side indentation in the rear floor storage area.
After test-fitting I found the one downside - it does not fit the front wheels, because it does not clear the front brakes!!! It fits with no clearance problems on the back wheels.
That being said, if anyone finds either another Toyota or Chrysler compact spare that fits better, let us know!! But since the last time I actually had to use a spare tire was about 18 years ago, I'll stick $25 cheapo Chrysler tire in the back for now and just rotate the tires if I get a flat on the front...