The stand has four shelves.  The top shelf provides storage, support for the top two light fixtures and for an electrical receptacle box that will provide power to those two lights.  The next two shelves support waterproof bins containing the seedlings, support for the remaining light fixtures and for similar electrical receptacle boxes.  The bottom shelf only provides support for a bin containing seedlings.  The three receptacle boxes are wired through conduit to a fourth, distribution, box mounted on the rear of the second shelf, as shown in this figure.

Since the receptacle boxes have to be mounted on the bottom of the shelves, and the bins sit on the top of those same shelves, it is important to provide a means of attachment for the boxes that does not provide any sharp protrusions, which could cut into the soft plastic of the heavy, water and seedling bearing bins.  The method used, and described below, does not require any protruding nuts or fixtures. This was achieved by drilling and tapping a 1/8" thick mild steel strap to mate up with holes in the base of the receptacle and distribution boxes, placing the strap on the side of the shelf opposite from the box, and then using stainless steel 10-32 machine screws, passed through the holes in the boxes and the shelves to attach the strap and box firmly to the shelf.  Following this, the protruding machine screws were cut off flush with the surface of the strap. The result is shown in the next figure.

To make the strap used above, use a box to mark off pieces of strapping equal to the width of the box, and mark the center of the holes with a marking pen as shown in the next figure.  Next. use a hammer and a center punch to mark the center of the holes and prepare the strap for drilling. Drill all the holes with a number 21 drill, then tap the holes using a 10-32 tap and tap wrench.  To do this, it is best to mount the strap such that the tap will be vertical.  Cutting oil will make the job easier and faster.  Cutting the strap into lengths equal to the box width is most easily done with a high speed abrasive cutting wheel, as shown, but is also easily done with a hacksaw.  Either method leaves sharp burrs, which should be removed with a file or bench grinder.
 
 

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