Extension ring for 4" x 2" welded "handy box"?

I ended up putting a layer of 1/2" OSB before the drywall on a wall to allow the secure installation of shelves, but now that I have everything ready to to install the outlets I realize that the box is a half inch too far back into the wall. There are two 4"x2" welded handy boxes in this wall that I need to add some sort of extension to, however the only handy box extensions I have seen are 1-1/2" or 1-7/8" deep. Does any know any other options?