I've wondered about these snap-in caps from McMaster-Carr for years, so I finally bought a box of them. They work great and look nice -- the pics below really don't do them justice. They differ from the originals as follows:
The finish is matte nickel-plated steel instead of chrome. Great candidates for paint.
They sit pretty much flush -- the originals sit about 3/16" proud.
$6 shipped with tracking. Make PayPal payment "To A Friend" to:
william@handspiral.com
and be sure to PM me your address.