Compare it to some other matteboxes:
mediahalo.com/equipment/matte-box/?vim

The RJ Matte box technically isn't bad, because it's exactly what you should expect for under $40(I now see if for as low as $15!), and what you get for that price isn't available for even close to that amount anywhere else. So despite plenty of issues i have it, i consider it a useful purchase i know i'll have work for it. Consider it a very cheap alternative to using an external flag, tape, duvatine, or whatever else you tend to use to get rid of a stray bit of light getting into your lens

Buy link:
amazon.com/dp/B0067O2VAA/?tag=viewnow-20

Same thing, different badge:
amazon.com/dp/B0069WHUO2/?tag=viewnow-20

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