Yeah, this is really annoying. Also, “left” and “right” are different for stormglass than for other building materials.
When you select a “Stormglass Left-Sloping Inverted Wall”, this is what you’re placing:
Contrast that with, for example, “Turanian Left-Sloping Inverted Wall”, which gives you this:
Both are “left-sloping inverted” walls.
That said, I’ve been able to place them more or less correctly if I put up a ceiling and hang them from the underside of that ceiling, as long as there’s no stair, ramp, or sloping roof where the wall piece is going to go. Those can be added later without problem, but if you put them down first, then you’ll have the problem @BradMunkey mentioned.