I've used wallpaper or scrapbooking paper attached with a glue stick to tag board cut to size. I bought some matting board on Ebay and that works great. Our church sponsored a tour of gardens this summer along with a craft sale. I took an old hymnal and took familiar songs, burned around the edges (some left plain) and mounted it on the backing and glued dried flowers around the song. They were a big hit. Used 8x10 frames.

here's a good how-to, hope this helps...

Here is a link that might be useful: sealing a frame