First, a plywood box to hold my laptop at a good angle to be visible while standing. As a bonus, a mount for the phone as well:
The plywood box (which is two sheets of plywood screwed to a thin strip, with no sides) is screwed to a Kee Klamp M58 Base Plate, which in turn is bolted, with locking washers sandwiched in there, to one flange of an F50 Female Single Swivel Socket, though I now realize a MH50 or M50 would have been simpler, since the second flange is useless for this application and makes it a bit trickier to assemble. The whole thing goes onto a short piece of pipe, which then mounts to an A10 Single Socket Tee on one of the front pipes.
I also put up another small shelf, similar to the mouse pad and using a similar combination of Kee Klamps, on the opposite side of the keyboard from the mouse pad.
Here's how the F58, M50, and plywood all connect.
The speaker had a mounting screw hole at the top, but that's intended to hold the speaker against a wall. Without a flat surface behind it, the speaker wouldn't hang properly. A strip of plywood screwed to a Kee Klamp 70-5 Rail Support fixed that. It also got me wondering why I didn't use the 70-5 instead of the M58 + M50 combo for the four other places with plywood (keyboard tray, mouse tray, laptop box, and shelf). There is actually a reason: the front poles are sloped around thirty degrees, and the trays need to be positioned and level at arbitrary locations in space to line up with the ends of my arms, whereas the rear pole that the speaker is attached to is almost vertical and the speaker simply has to hang in space.