For streamers, nymphs, wets and such, I use a store bought frame and cut a window matt with two windows sized to suit. Unless you have experience and the tools to cut matts it is easier to just go to any good custom frame shop. A good one will work with you in choice of matt, frame and will cut the matt to suite. I hold the fly in place with a band of fine tippet looped over the shank of the hook and run through two pinholes in the backing matt board, and then tied in place.
AgMD