I did it! I made some simple saint costumes using pillowcases thanks to
Catholic Icing and
Shower of Roses. Another suggestion from Lacy at CI was to use a t-shirt that is turned inside out.

If you cut out the neckhole just right, you get a little St. Francis cap, too.

Here they are: Blessed Kateri, my little angel, and St. Dominic.
I'm already planning for next year.