And that's when I realized that I am terrible at writing anything longer than a paragraph.
This is not actually true.
I made it a joke because at least one other one was similar but not the same as this (that being SCP-1049-J).
Obviously, this is a concept that would never make the cut as a serious SCP object. I made it a -J precisely because of that.
But if people don't like it, into the brain-trash it goes, like so many other things.
I don't think the problem you're having is because of the writing necessarily. That's at least not the main reason that people are saying no. I think the problem was more that the basic concept kind of, you know, didn't make sense. There is no "opposite" of glass. You could have made the object something that would have turned things into their opposites with respect to certain specific details, like you could have an SCP that turned transparent objects to opaque ones and vice-versa, but you can't decide that any object is the opposite of another, because there are always more aspects to be considered. Glass and iron are opposites with respect to light transmission, but not when it comes to phase of matter (both solids), mass/weight (both have mass), matter/antimatter (both are matter), whether they're animate (both are inanimate) and so forth. Pretty much any physical, real-world person/place/thing is going to have the problem of not having an opposite, because oppositeness is an abstract quality.
And the problem with making it a -J is that it didn't (apparently) strike anybody as funny. Which, jokes not landing the way they were intended to is something that happens all the time; it's not a sign that you're a terrible writer, just that you're a person who attempted to write something funny. Learn from it and go on.
Glass was the most fragile material I could think of that doesn't collapse under its own weight. And I later made it less specific: A locked box of a more durable material.
The joke is just "What happens when we oppositify certain people, objects and SCPs?" Apparently, nobody likes that, hence the removal from the sandbox.
Just a newb here, but at this point is it safe to say, "[REDACTED]"?