I have a very similar unit - an older Sears model made by DeVilbus. You are not showing a metal plate on the top that is held by 2 bolts into the threaded mounting bosses. The plate holds the thick fabric filter mounted horizontally and encloses the airbox. Try this Sears part number 265-17. Grainger very well could have it also. Plate could be fabricated or check a good compressor store.