Your question is a bit unclear. Are you just asking for recommendations on which graphics to use for such a block, or how to execute the block itself?
I'm going to assume you mean the former, considering this forum. I don't know if there's any "standard" answer to this question, but it really shouldn't matter as the level/hack you're designing introduces the level in an intuitive way such that the player has no difficulty figuring out the block's function - if you don't introduce it the right way, then the player will get confused no matter what graphics you use.
Personally, I'd probably go with one of those "empty outline" blocks (i.e. the ones that the Switch Palace blocks use), except maybe give it a unique palette to distinguish it from other blocks in the level.
(If I misunderstood your question and you're looking for the code, it's )
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