Threaded Inserts

ScopeRoller uses casters with two different stem sizes: 3/8"-16 and 1/2"-13.  The smaller size is for the less expensive (much less expensive!) low-priced version.  It would be good to have a single size threaded hole so that I can make up dozens of the threaded inserts at once.  One alternative that I have used once to retrofit an existing threaded hole is what is called a threaded insert, which looks like this:

The external thread is 1/2"-13; the internal thread is 3/8"-16.  You turn it into your 1/2"-13 threaded hole with a screwdriver, then screw the 3/8"-16 stem into the interior.  This lets you use one size threaded hole for both stem sizes.

There are many variants of this.  Some have little keys that you push inside to lock the external threads into the hole:

This prevents unscrewing the smaller thread from unscrewing the insert.

The problem is price.  The only reasonably priced ones (under $1 each) that I  have found so far are these:

These are brass with what are called "knife edge" threads, intended for self-tapping into wood.  (Very popular with home brewers, apparently.)  This might work okay in acetal, but at some point I want to switch to aluminum, and these threads apparently break under stress.  I really don't need brass, either.  But I am not having any luck finding them under $1.

Rockler seems to have what I need. $6.99 for a pack of 8.  Back ordered right now.