First of all, here's a good guide:
http://users.animanga.com/kallred/FrameIt.html

What I use is acid-free mat board. You can 
either hollow out a piece as it describes in 
Step 3 or another thing is that you can assemble 
it from leftover strips of mat board and acid-
free tape (less waste) since the spacer won't be
seen anyways.

Hope that helps.