I am planning on building a block that is about 16" square to mount an exterior light. The block is rimmed with a 4" casing (sometimes called a "molding"). The casing is 3-7/8" wide and 1" high at the outside edge. So, the casing will form sort of a picture-frame-like rim around the block.
The problem is fitting it together so that no fittings are visible and the miter joints do not open up later. I want to minimize exposed screws or nails that would have to be covered up somehow.
In old books I have seen that when similar types of joints are made they recommend both using a tenon of some type to register the miter and also a screw to tighten the miter. The problem with using screws is that it would leave holes in the outer rim of lamp block.
I know that with picture frames they staple the miter on the back, but that seems too flimsy for this use.
Another idea might be to use flat brackets as shown below, but the problem with that is that it will stick out from the back of the casing, so some kind of recess would have to be milled, so that the casing can sit flush on the base block. Also, a bracket like this will just hold the joint, not tighten it.