I like the stackable shelves, they allow me to do several flats of seedlings at the same time. The biggest drawback I have on the shelves is they are not adjustable. The plastic tubes are a specific length and you cannot adjust them without cutting the tubes.
So I studied the shelves and found a way to make them adjustable. I cannot say this is true on all shelves but on the sets I have purchased, where the tubes pop into the corner of the shelves is an opening. So the tubes actually rest on a thin lip of plastic.
Looking at this, if the tubes were narrower, you could move the shelves up and down. So I thought what could I use for the tubes. I love hardware or home improvement stores, and in my wandering, I came across white PVC pipes. They are stiff enough to make not only shelving but I have seen complete greenhouses made from them. They would work well for my shelving.
I had to do some adjustments though as the opening in the corners was not large enough to accomodate the pipe I purchased. So you might want to measure before you buy, but if you don't and you do as I did, there is an easy fix. You can grind out the plastic to fit the size of your pipe with either a file or a drill bit.
Then I had to find a way to make them adjustable. Most shelving units today have little wooden or metal pegs that hold the shelves, so I wanted something similar. I have purchased wooden skewers from the dollar store for many uses but they have come in really handy for this. I measured out increments on each of the pipes I used and then drilled holes with a bit just larger than my skewers. Now I had an adjustable shelf system.
Now I had to find a way to light it.