I put too much junk in my closet and the shelving collapsed. The brackets holding the shelf to the wall shattered. In an effort to save $5.99 plus shipping I figured I'd make one myself

In the interest of structural integrity, you should probably print with 100% infill. However, it should be noted that a printed part won't be as strong as a factory made piece. Don't actually use this bracket if you plan on having it support a lot of weight as I haven't actually tested how durable this print is.