I'd cut a piece of 3/4 inch plywood to the outside dimensions of the rails and notch out the four corners for the steel pillars. Clamp it in place and check for level, etc. Also put your printer there and check you have enough room to open the lid and feed it paper etc. Be happy with the position before you move on.

Then with a small bit, drill from the sides a hole through the pillars and into the wood. You're only trying to make registration marks with this first pass, but do it carefully so everything lines up. Get four suitably sized bolts (they need to be long enough to go through the metal pillar and still have an inch or so for the plywood), and take one to the hardware store. You need a drill bit which is the inside diameter of the threads, a drill bit which is the outside diameter of the threads, and a tap which matches the threads.

Drill the pillars out to the large size and the plywood to the smaller size. Tap the plywood with an electric drill on it's lowest setting. Back the drill up every turn or so to break the chip. Be careful not to go deeper than the hole you drilled or you'll wreck your threads.

If you don't want to start a collection of expensive taps and want to buy the most useful tap size, I recommend 1/4-20.

Now you can just assemble with the bolts. Should hold 150 pounds or so, depending on how thick those steel legs are.

I would probably make a one piece shelf with notches in the corners, then use four brackets made of pieces of angle iron the same width as the legs. A single 1/4 inch bolt drilled through each leg and each corner of the shelf would likely be plenty strong. If you need adjustment, drill multiple holes through the legs. (Though shelves have a way of being adjusted once then left forever at the original height)

Could perhaps make a jig and row drill each leg, using cabinet shelf clips

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