Here's the type I use, they're pretty well made so I guess Leviton also has a cheaper version: T
There's also a version with the starter block attached to the bottom. These are basically the screw mounted sockets that vintage preheaters used.
Hey Mike, the doughnut ones I can get are identical to
this except that there's a nut in the slot in the middle that lets you screw the socket down.
I find most lights have some play in the socket slot and if you move the sockets the furthest apart the lamps are loose fitting and tight fitting if you move the sockets closer.