I would go to a hardware or plumbing supply store, and get some kind of metal tubing that will just fit in the hole.
I think some have used copper, but I think aluminum might be better.
I bought my behr locally from my mechanic last year. it had the reinforced neck. Check the DIY page for w124 radiator and you can see pics showing it.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.