Even with a lathe I'd be temped to centre drill stock 50mm stainless bar pre-cut to length, bore out (or drill) then ream to 15mm dia and maybe 40mm depth, then tap some 15mm dowel into the reamed holes with some loctite.

I wouldn't bother trying to turn down the 50mm bar. At least with my ML7 (small spindle bore) I'd have to turn between centres.

I guess there's a reason it has to be stainless?