What about the folding gas block sight that looks like a standard FSB? I think GG&G or ARMS makes it. I know they aren't held in the highest regard in the industry but they have some unique products that fit a few niches. This may be one such application.
If I recall correctly, it doesn't lock either. Not sure how it holds up under impact as I've never handled it.
Not sure what rifle you're using this on or the rail system you're using, so it may not be an option but something to look into.
ETA: http://swfa.com/ARMS-41-B-Silhouette...ht-P40334.aspx



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