Tons of really good, practical ideas in this thread. This is why I love this place.
If absolutely all you need is a T-nut shaped thing for your tailstock, then any of the simpler ideas will do you absolutely fine. Crack on and get your part made.
That said, getting an ER32 collet chuck, a few usefully sized collets and endmills to suit, and a milling attachment does have some benefits beyond your current goal.
It's a fairly cheap (ER32 collet chucks for mini-lathes are fairly inexpensive and accessible, as are milling attachments) and simple way of adding some limited (I say limited because you won't be hogging out huge chunks of steel with this kind of setup, the rigidity just isn't there) milling to your 'toolkit' without having to buy a milling machine.
It'll enable you to tackle other projects which, without some kind of milling capability, will be either very tedious, difficult or in some cases virtually impossible.
And you'll learn some stuff and have a bit of fun whilst you're at it.
As I said, if you're happy going the hand-tool, more basic type way then go for it but I just thought I'd raise the idea up the flag and see if you had any saluting urges.