I've seen it done several ways. The quickest is to just use a common machine screw 'nut' and they are often inletted into the wood just under the top tang and epoxy glued in place.
You could reverse taper the flats of the nut slightly so the glue imbeds the nut more securely. It's still just the epoxy and it's secure grip on the surrounding wood that holds it in place.
Placing it on the underside gives it a lot better strength. It doesn't even really need to be 'glued into place if the nut fits it's inlet tight enough not to drop out while assembling, but it makes life simpler.
A bushing will get you the max amt of thread engagement betw the two fasteners. But the bushing's grip on the stock is still dependent on the wood being oil free for the epoxy.
In this case the new stock elliminates that issue.
A thin walled internally threaded bushing with a flange on the bottom to pull on the wood as the screw is tightened might be best. Maybe overkill but as long as you are making a bushing it's not hard to add that feature.
Lots of ways to handle the Flues top tang screw problem.
