This framed Christmas scene is 12" long and 8" high. The backer board is covered with Christmas wrapping paper to make the wood cut stand out. The frame is just a simple butt joint assembly. You can use 1/2" or 3/4" thick wood but I think it looks best if the frame is 3/4" and the cut out is 1/2" thick. That gives it a shadowbox effect.
You can use cardboard for the backer. Cover it with the wrapping paper and staple it to the back of the frame.