Thanks. Got the store and price. On page four of your tool link they've got the tool, using this description.

Mercedes Reamer
number 901-0053

This "Pencil Type " Reamer ( 12 x 1.25 ) is used for removing carbon buildup
in Diesel Glow Plugs which is a major cause of glow plug failure.
Same as Mercedes number 901-589-0053-00
Applicable: OM 601, 615, 616, and 617
$39.99


While its worth more than forty to fix the problem, the tool looks like something that a homemade version might exist.