Besides walls/floors, the TIM games have some square-ish filler parts that function as walls/floors, unaffected by gravity, and that also don't do anything else. There is a metal cage, a kind of box made of planks, an archway, and perhaps more I didn't think of. (I really should play through the TIM Mega Pack I bought on GOG.com a while back and refresh my memory.) These parts made it possible to create puzzles that looked a bit different, and they were useful to fill some otherwise empty space.
Could something like these be added to Contraption Maker? Even better would be if you could add any rectangular size, let us drag the borders/corners to determine the shape. Maybe not just rectangles, but also other polygons? I could add stack a bunch of brick floors if I wanted to make a house, but that doesn't look as good as it would if it was all one thing. (Also I've noticed they don't seem to always snap to the same grid, or at least are offset by a few pixels sometimes.) Thanks for listening!