Not specific size/layout you posted, but what about one of these?
http://www.homedepot.com/Building-Ma...atalogId=10053
http://www.homedepot.com/Building-Ma...atalogId=10053
Standard corner braces, one at 2 inches long, the other at 1.5. Generally speaking, any hardware store is going to carry some corner brackets (usually near the door knobs)... they may not be exactly what you need, but if you can make them work...