3D work can look great matted, especially a small
piece you want in a larger presentation format.
One quick way (of many ways) is to cut the mat opening
out of a board that is the mat size + the desired
depth on all 4 sides(depth of the frame minus glass
and backing thickness of course). Then score (from the
front) the desired mat size - (minus a tiny bit to
account for mat thickness). Go all the way to the edge
of the board when you do this. This will leave squares
in the corners of the mat board - as you say, about 3"
x 3". Cut out these squares out, bend the sides back
and tape with linen tape. Then you can pop this in to
the frame after the glass. This is basically the
reverse of building a matboard box for a shadowbox
frame (when you want the sides of the shadowbox to
match the mat color). The sides will support the mat
as it floats over the #D object.
I have seen artist-jewelry done this way without
glass, so the piece can be displayed when not being