You can almost take your pick of materials. I've seen some carved from hardwood and use several coats of finish to avoid wood grain issues.
But personally, because making a plug usually involves a lot of tedious sanding that I want to go as quickly as possible, I like to work with softer materials; and then fiberglass over them for strength. Lastly, I use Klasscote Grey Primer for a durable finish that stands up to moldmaking.
Schedule 40 blue foam, or similar foams work well, and is relatively inexpensive. You can go to the "P-39 Anything thread" and see a foam plug, mold, and cowl (as well as a fuse plug) that I made using this process, starting on post # 170. Here is the link:
(The foam I used for this build wasCyano-somethin'-somethin'
. . . way too long a chemical name for me to remember at this point, and way too expensive anyway! Not worth it when there's Schedule 40 blue foam. My cowl plug was already painted with grey Klasscote primer . . . sorry . . . but, it looked the same as the fuse does in the following set of pics, before paint.)
I've also had great results making my cowl plugs from balsa (and covering with fiberglass, etc.).
Another method to consider:
If you are not trying to go super-scale or get overly meticulous, you might even consider just taking the cowl you have (if it isn't all cut up, already) and glue balsa sheet (thicknes dependent on whatever extra length you need) to the trailing edge, and carefully sand to shape. Fill &/or fiberglass with light weight cloth, primer with grey Klasscote, sand/polish some more, and there you go!
I'm not familiar with what a Dewey looks like, and therefore what the shape of the cowl would be, but in some cases I've extended the length of the cowl by the same method mentioned in the last paragraph, but by gluing the balsa sheet to extend the nose ring (of an in-line engine design), rather than at the rear.
These methods work well if you only need about an extra 1/4" or so, and the cowl wasn't designed with extreme angles.