I recently upgraded my from my early 2011 MacBook Pro to a 2017 iMac, which meant moving the ultrawide display to where the MacBook Pro used to live. The problem is that I had previously built a desk-mounted shelf and desktop speakers stands based on the design of the size of the MacBook Pro.
The solution was to simply make new ones. I built two new speaker stands that stood a few inches over the top of the monitor. The design is basically the same as my old one, but with some slight differences due to the metal stock available to me at the hardware store I went to this time. For example, the vertical pieces are smaller square stock, as are the L brackets that the speaker rests in, which necessitated a crosspiece underneath the L pieces so that there was enough room for the speaker. One of the verticals wasn't exactly vertical once I'd welded it, but this was easily fixed by applying some heat to the base and just pulling it from the top until it was properly aligned.
For the shelf brackets, I decided to use the same clamp design as before (a piece of square stock with one side cut off to make a "C", with a rivnut through the bottom to screw it onto the desk), but with a second shelf support about half way up pointing out the back, thus allowing me to add a second shelf and balance the weight. I had to (sloppily) weld a short piece of L bracket underneath those lower brackets so that they wouldn't bend once the shelf was on them. I omitted the small lips at the ends to keep the glass shelves from slipping off, as I don't think they were needed and weren't worth the effort.
That's it. I'm pretty happy with the results, and now I have a bit more space for toys.