I am a scrapbooker, too. Have you seen the scrapbook frames at Micheales? They fit the 12x12's great. they run 9.99 but are on sale a lot for 5.00 each. Also, a sheet of glass for a frame isn't too expensive. Have you looked at Hobby lobby or Micheals where they will cut the glass to size? I had this done for an odd sized frame and it wasn't very expensive.
I am afraid if you use plexiglass it's going to look cheap and it might start to warp over time. I have a frame that is of plastic, big poster size and everytime I look at it, all I see is my cheapness. I need to replace it.
since it is your family photos I would perserve them as much as possible.
Good luck!
T. in Allen