If you're really adventurous, the 5/16" Lucite stems from
Pipemaker's Emporium are just about made to fit on the Missouri Meerschaum Cobs (Only the Cobs with Vulcanite and filtered bits, the regular un-filtered bits are slimmer).
Lucite Stem #491
5/16" X 4 1/2" Fancy
$4.75 each
Technically all you have to do is sand a few thousandths of an inch off the tenon.
The stem only comes with a 1/16" draft though so widening that is probably the biggest part of the operation. Drilling a 1/8" draft hole shouldn't be too hard.
The shank on the Cob, however, is actually 3/16". I have a tapered tip drill bit that's just 1/64" smaller, which should be ideal, but on my few lazy attempts I've only blown up the stems thus far, if you want the full classic MM draw it's going to be more difficult.
You also get to learn how hard it is cutting the slot in the button (the stems have a shallow slot by default, but I don't settle for anything less than an inch deep).
Note that the Vulcanite stems you can get from Missouri Meerschaum also have a much slimmer draft hole than the plastic stems, so you're not saving any time with those (unless you just want a pre-fit regular Vulcanite stem. I have a few, they're nice).