I haven't needed it, hopefully I never will, but in my YouTube journeys I have seen this.

Seems to me that with it that deep a guy could get a bolt to run in there.
Start with the bolt cut off to stick out enough to be able to screw it all the way down and still get it out.

Get a hole down the middle of it. If that means a visit to a machine shop,, mehh, twenty bucks or less..
Screw it in, use the hole to guide the drill. Drill into the dutchman. Follow it up with a bit an easy out will go in.
If I didn't have a mig, that sounds like a fair idea.

Paul? You gettin this?