Mix a small batch of plaster and just a little thin in consistency. Use it as thick glue or mortar to set the piece on the frame. Gently push the piece into place. The excess plaster will ooze out around the edges. With a wet finger smooth around the seams and remove excess. Don't try to fill all the seam joints at this stage. Concentrate on getting a perfect fit. After the piece is set and dried it will be easy to add a little extra plaster to fill any gaps. Do a little light sanding with 240 around the seams to make them blend. Dust off with light air pressure and seal the plaster with a coat or lacquer or shellac.