Being a flyfisherperson, I''ve framed lots of flies. A bent over "T-Pin" hot glued to the back side of a mat works well for the post. A dab of crazy glue on the point and impale the fly on that.
I''ve also used plastic micro tubing cut to length and bonded in the mat. A monofiliment looped around the fly and hot glued on the back is more archival but harder to do.
I did 2 16 x 20 1" deep shadow boxes with 98 flies in them. Never again! It turned out beautiful and the customer was pleased. Should have charged by the hour.
Making the shdow box was quick and easy with FrameSpace #5 (3/4" airspace.