Go to Home Depot or any craft store or even Walmart (as much as I hate that place) and buy picture hangers since you say it's essentially a big picture frame. They have them for different weight frames.

It's made of pine and is essentially a really big picture frame. It's about 5' X 4'. It's depth is about an inch and a half. I tried finding some shelf brackets but I couldn't find any shallow enough. They all would extend well beyond the frame.