spsoe it depends on what your making your adaptors out off?? alloy yeah need and insert as you said as there gonna strip easy with a bit of ham fisted ness... lol... mild steel would be ok pending on the thickness , but who wants manky rusty / painted ??? if you get em custom made id go for stainless ones, no helicoils, nice and shiney/never rust.....
can get em made at my work in stainless.... at a cost of course...