I've been at it again. When I did that I-beam some time ago, I made a desk shelf that I'm still using in my office. The idea behind it is that a small shelf can sit on a desk and actually give you more surface area because you have an accessible second level. Clearly, there are diminishing returns. If you covered your desk with them, it wouldn't work so well. One or two, though, can actually help. You can see from the picture that from the year or so I've been at 3d printing, I've accumulated a lot of objects. My wife, out of frustration, threw them in a box. She seems to like the desk shelf though.

Print the parts, 2 of each, and fit them together. They actually can fit together in more than one way. I designed front and back supports to maximize the space in the front. Also, in the prototype (seen in the pictures), I messed up the length of two of the corners. Logic being what it is, it didn't matter as the size difference could be made up by properly matching the parts. I resized everything for the upload. Let me know if there are problems. My first iteration required a lot of sanding for the connectors. I've widened them up a bit, but you will still need to sand a little. Glue felt to the top and bottom for better aesthetics.