AR15 Lower Receiver Painting Stand
thingiverse
The name gives you the lowdown. You'll need a 1/4-28 screw (mine are about 2.5 inches) to lock in the lower securely. I had this custom-made before I could print my own stuff and ended up drilling out the hole for the screw. Scaling it up a bit should fix that issue, but I'm not sure how much.
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