I like your leatherheads
I had a hard time visualizing them when I read UTD. I ended up thinking their appearance might be similar to an origami fortune teller, which is sorta kinda how you've illustrated them
Cool! Great minds, you know.
But seriously, in my mind I can visualize them in so many different ways, but I cannot draw any of them... I can see them as changing all the time, as if our minds could not lock into any stable way of interpreting what it is "seeing". Lots of lines (of energy, perhaps) coming out from everywhere, as a diagram of a magnetic field. Perhaps even as a simple cylinder with two diamonds at the side. When we are told it hurts to see those sort of creatures (I dunno if N. or It had something like that), I think it is because our minds are not made to understand higher dimensions.
One way of getting that may be that each character sees them as something different, and the similarities only appear when they compare observations. I was thinking of doing Norrie's version of this pic, and to have her see them as actual (human-like) kids. That way the goggles and the hats would be more evident (she was the one who had less trouble describing them, and she said "hats" instead of the "helmet-shape" Rusty saw). Even though Rusty had thought about kids when he saw them, she is the one who actually said (out loud) they were kids without any hesitation.
I don't remember where I saw the following idea. It could have been on these boards, or on someone else's fan art, but I really liked it (I did some searches yesterday to see if the idea came from any official source, but I could not find anything). That idea has the leatherheads have some ant-like heads. I can totally see that because of the shape an ant's head has, as it can be interpreted as these geometric solids. Plus, there is the whole human-ant relationship present, too.
And now you see why I had such a hard time coming up with something to represent them...