The images above are a few useful pieces of hardware from Mcmaster Carr. You can find them all by using the keyword angle bracket. The first I have used quite a bit on my last show. The second I have used for when something needed to be removable - I have also used "picture hangers" from other sources as well. The third, called zee-clips, is an often used piece of hardware here to attach pieces together that are double sided or that attach flush to a wall. They can also be manufactured according to your own specs in the shop. Sometimes we use something more circular that we call pucks - and then route a keystone into the pairing surface. French cleats - where you cut a 45 degree angle on each piece and they mount into each other works nicely as it draws the 2 surfaces together.