RR, it is 3 sided. Consists of 1 piece that is bent at 90 degrees and then one piece for the other side. The front on the bent piece is lower than the other 2 sides as you can see if the pics. The back is open to the air(and dirt if your fender is short). The sides measure 6.5" top to bottom and 5" front to back. the front section measures 4" side to side and 6" top to bottom - you can see the drop down angle in the pics. The body metal is 1/16" and the interior brackets and the lower bracket(for the single side bottom to attach to) are 1/8" thick - they are all welded on. The lower bracket that is welded onto the bent piece is 1x4". The holes are 5/16" and line up with the holes on the bottom of the oil tank and the hole in the bottom bracket is centered.
Pretty simply made to contain the regulator, rectifier and starter selenoid. Would have been cool if there was enough room for coils, but AMEN had that coil box that mounted on the front down tubes - which makes it tough to get the wires on and off the plugs.

I'm old enough to know better.... yeah right, like that's ever gonna happen