If you use silicone, wouldn't it be hard, if not impossible, to remove it? Seems like a lot of effort to just separate some axolotls for a few months.
If you're looking for inexpensive alternatives, you can go to a hardware store and get some sort of mesh. Cut it to size, tape it at the top and use rocks to keep it down at the bottom.
I bought my axolotls a tank divider when I first got them, but it's a huge waste of money. It's a cheap piece of plastic, might as well just make one. Sure it won't look as good, but the divider itself doesn't look good. Anyway, it's only for a few months.