I have a KAC VFG that I'd like to use the hole in the top to secure it to a rail rather than the normal method with the plug.  Clearly it has been designed for this alternative method of attaching it.  While I could just get a screw and thread it through with a bolt on the other end, I'd prefer a cleaner look.

I'm guessing the best way to do that is to use a proper-length screw (30-35mm) and countersink the bolt head and nut on both sides.  The bolt head is round, which isn't an issue - but how do I make a hexagonal countersunk cut on the other side for the nut?

This video shows how. You do have to clean out the waste metal often to let the broach cut properly as you basically have a mostly blind hole instead of a through hole. There are a couple other video's on that channel about doing broaching you may also want to view.

Interesting...  so this would actually be into a hard plastic rather than metal - and while I have tools like an inexpensive drill press, I was hoping that there'd be a way to do this without spending a lot of money since I only need to do it once.  

Am I relegated to just using some hand tools like a dremel attachment to approximate the size to cut out and 'eyeballing' it using a steady hand?  

For plastic I think a narrow wood chisel, exact width needed or smaller, would work better than a dremel. Use the nut and bolt (use longer bolt if necessary) to center and hold nut in proper position, then scribe outline, remove nut and bolt and use wood chisel to cut the hexagon shape into the plastic. Hopefully hand pressure would be enough to shave out the plastic, but a mallet could be used with care.

That's a good idea...  didn't think of that.  Of course, I'd have to find a chisel small enough for that.  

Thank you.

Maybe put the nut on a spare bolt, heat nut with torch, then press it into the plastic? Repeat as necessary to get desired depth.

Maybe you could drill circular holes on both ends and use a star washer like this on the blind side?
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