I have just finished this sampler. I constructed a deep box frame to hold the 3-D embroidery and this technique could be used for other projects that need a deep box frame. Ive used this technique to make miniature rooms - miniatures
I used a bought photo frame and cut out the sides of the box from stiff cardboard. Using kraft paper tape I joined the sides to the backing of the photo frame. You need to stabilise the box by applying kraft tape to the inside and outside of the box. Then its up to you how to mount your embroidery. I used a backing card the same size as the back of the frame on which to mount this embroidery. I lined the four internal sides of the box with strips of the same linen mounted on thin card cut to size.
I hope this helps to give you more ideas on finishing. Have fun!!!