See Rewrite the Future: Could white have this sort of counter?

Hmm. Well, certainly needs reminder text; and probably templating (manifest a card, not a spell; but meh, the intention is clear).

This is... probably slightly bendy but ok? It's a tax, which white gets to do. It's a bend because white usually has to announce its taxes in advance, not spring them as a surprise.

It does not need reminder text; it is clearly enough how it is. (I did make up a similar card myself, with a different mana cost and color, and without "unless its controller pays ".)

For my own card set, I also made up the rule "non-card objects moving from the stack to the battlefield change into tokens" which may be official now, although I do not know if it is actually the same rule or not. At least the way I did it would allow copied spells to be manifested as tokens, but I don't know if the official rules do that (or if it is only for resolving spells) because the new rules have not been published yet. I try to always pay attention to changes to the Comprehensive Rules when they are published, in order to avoid missing such things like this, but I have not seen it yet.

I've also created Solidify, which is the hard version of this and doesn't work around "This can't be countered."

I think it's most likely that attempting to manifest a copy of a spell would just make nothing happen, but I could be incorrect.

I think a more white counter would be a seal, even if it was a hard counter, but where it's all explicit and up front. If Wizards are extending counters into white and black though a taxing soft counter is ones that would make sense to appear first.

I like the rules elegance of "manifest target spell", although I suspect that may make it too Melvin for most players :) I did need reminder text, I needed to look up that you got the creature not the controller of the spell.