Delrin  comes in many sizes but plastic suppliers don't always stock all available.

Thin ones are wippy for sure. Home cleaning I prefer wood for anything 40 or up, or if a smaller bore Steel. 
I use Steel in my .25 calibers. Have a 1/4 inch Delrin that works fine in .32 caliber at the firing line.

You could take a 1/4 inch Oak rod chuck it up in the lathe and sand it down to fit a .25, does not take much off to fit.  One of my Marlin CLA Silhouette rifles will fail to extract at times, Have a 6 inch 1/2 inch brass rod cut down just a bit to be a slip fit. Drop it in the muzzle it knocks the empty out fast during a relay.

The Delrin rod has the advantage of working around a tang sight or high comb, that's why I use one in my 32/40 Schuetzen.

If I had to buy them would only have a few rods. Making them have a rack celling of my shop with a dozen or more each a special job.

Boats

NC that 5/16 inch White is what I use too. Just measured it's not 1/4 like I said before. Makes a good rod, bends a bit in the bore but all I use it for is a quick wipe. Hard work use steel and a bore guide.

Boats