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You can get a few different designs of RDF. Some of them have an enclosed reflector, some it is exposed, and some like the Telrad have it angled.
The gun sight RDFs from Taiwan can be had fairly cheaply, and they commonly have a selector wheel for different reticle types and brightnesses. Note that they are commonly too bright even on the lowest setting once it gets truly dark, as they were not designed for astro use. This type of RDF can be bought from GSO, WO, and Vixen if I recall, among others.
Baader, Televue, Celestron and others also have semi-enclosed designs, where the reflector is inside of a dew shield. These ones are designed more for astronomy use, and while I have not used one I will likely switch to one from the above type at some point.
Make sure that whatever you get, it is stable and easily adjustable, and goes as dim as you need.
Regards,
Eric
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