You can use cloth easily...just glue it around your cardboard backing. I've used silk, or burlap, depending on the plants used and whether the effect I want is "formal" or "fun". You can get some marvelous effects playing around with fabric.
Another idea is to use wallpaper samples...some have a suede like finish, others a satiny look, in colors from ivory to deep brown (good for pale dried grass seedheads,for instance).

I've even used nylon pantyhose material in small frames...lovely taupe shades to compliment the colors of lighter colored flowers, or pale for deeper colored flowers. (I'm constantly looking for re-use...lace from old lingerie makes nice borders...smile).

Just play around and have fun. jo

Great ideas! I plan on trying some of these. Would have never thought of the pantyhose.

Just saw some framings on Kitty Barthalomeow and they were saying that the botanicals were used for medicinal purposes until some keen decorators saw how nice they looked and began using them. So that's how it all got started. Very exciting!