Yes, it’s plastic… no, I don’t expect it to hold up to extreme use (but you’d be surprised at how solid it is).
Without any mounting hardware, it fits tight on the rail and tight on the light. I picked up the basic design from Thingiverse.com and had to modify the diameter of the hole to fit my Surefire light. It’s perfect for my basic needs.
What I don’t know (yet) is how well it will actually work on my AR. Frankly, I think it will be fine – the light (which isn’t purpose-built as a weapon light) may have more issues than the mount. Based on what I’ve read, I expect the recoil to mess with the on/off of the light based on the spring loaded battery contact and the momentary switch.