Star Trek Engineering Crewman
This is a simple outfit: the uniform worn by the Engineering crew in the original Star Trek.
Thomas loves wearing this costume because of the effect it has on an audience. People know that the outfit is familiar, but they can't figure out where it comes from. Then, when he tells them, they always hit themselves on the head and say, "Of course!"
Below: Thomas on the bridge of the Enterprise, at Worldcon in 2006. The rest of the crew are waxworks.