I am curently rebuilding two CubeX printers for Stockholm Makerspace, retrofitted with Bondtech QR feeders and E3D V6 hotends, and realised when i got the feeder that i did not have any M5 inserts at home, so i designed a new mount for M3 inserts :-) I printed the parts in black Ultimaker Polycarbonate to get a bit overkill combination of strength and heat resistance, but almost any plastic should really work unless you will use it inside a heated chamber.